Selecting a species is mostly about matching the species characteristics to your own character and style of play. Currently, it is the general view that the species are well balanced. In other words, the only determinant on how well you can do in a galaxy is your skill and not the species you play. However, early phases of the game are dominated by players/species with good economies, while militaristic species generally have poor starting economies. As the game progresses, military advantages become more important, while economic advantages tend to be eroded. If you play with an economically strong species (Zeeb, Toag, Shy'dow) you want to make your best efforts early on, and establish a significant lead before the military powerhouses (Gorgoth, Turian, Kreel) get underway. Other species (Sirnef, Oomari, Drall, Ji) have more balanced attributes, allowing for varied styles.
Drall
Style: Expansionist, Militaristic.
Difficulty: Easy to start with, medium to hard difficulty in later stages.
COmment: Rapid pop-growth and a level 3 production technology to start with makes for a fast growing economy. The pop-growth also means you will never be short of population in the race to colonise planets. When the major wars start, Drall have no inherent military advantage (apart from cheap missile research). Capturing ships is not that easy, and rarely makes a difference in a single battle. You tend to capture badly damaged ships only, and more often than not you won't be happy with their design. Scrapping captured ships yields no salvage, and maintaining them costs you upkeep. Over an extended campaign though, you could capture sufficient ships to significantly add to your fleet tonnage.
Gorgoth
Style: Militaristic, Expansionist, Offensive.
Difficulty: Hard to start with, but progressively easier as the game progresses.
Comment: All economic techs are level 1 to start with, giving Gorgoth the weakest starting economy in the game. It takes a hardened economist to overcome this, but after that you will find that Gorgoths can colonise a wide variety of planets, and most species are good to medium hab for Gorgoth. With the most destructive ships in the game, you can tear apart any equal sized fleet, and conquer planets faster than your rivals. The 4% bonus to your market discount also makes Gorgoth an attractive ally.
Ji
Style: Expansionist, Militaristic, Offensive.
Difficulty: Medium to Easy.
Comment: Ji have mediocre economic techs to start with. However, Ji get a major discount to colonisation cost, and their colonies bioform twice as fast as any other species. This means you will have spare resources to start the game with, and you can build your economy faster because your colonies will be the first to reach 100 hab. In the early phases, concentrate more on population growth than economy, because your colonies will be cheap anyway. Throughout the game you will have resources to spare for ships. The Ji military bonus is relatively small, but equates to a high degree of efficiency in allocating damage to enemy ships.
Kreel
Style: Militaristic, Builder, Defensive.
Difficulty: Very hard to start with, medium difficulty as the game progresses.
Comment: Weak starting economy. The greatest hardship to Kreel is that habs are generally poor, meaning enormous colonisation costs. It takes a strong economist to play Kreel well. Militarily, Kreel ships have a superb advantage, in that they always fire first. This advantage is imparted to their alliance as well. Kreel military advantages extend to their minefields as well, making an assault on a heavily mined Kreel planet an arduous adventure at the best of times.
Oomari
Style: Expansionist, Militaristic, Guerilla.
Difficulty: very hard to start with, medium difficulty as the game progresses.
Comment: Weak starting economy. Oomari have two early hardships to overcome, bad habs and atrocious population growth. The Oomari advantage is most sharply evident on the strategic scale. Oomari and their allies can move from planet to planet in two ticks, allowing the alliance to expand 33% faster than an alliance without an Oomari. On the tactical level, Oomari ships have the potential to be very evasive in combat, but this advantage is easily neutralised by good enemy sensors, and further eroded as rapidly as the oomari ships take damage. For the evasiveness to be truly effective, the Oomari player has to make huge sacrifices in other areas of ship-design, which is extremely risky. Whether on the defense or offense, it is the Oomari who gets to choose when and where to fight, and intercepting an Oomari fleet is very difficult.
Shy'dow
Style: Expansionist, Militaristic.
Difficulty: Easy.
Comment: Strong starting economy, and cheap ore research plus bonus to ore-mining and reduced ore-depletion-rates ensures Shy'dow will always have a strong economic advantage. This is combined with the second fastest population growth-rate in the game (after Drall). Futhermore, Shy'dow ships are cheaper than most (discount to stealth, and engines), meaning you can build more, and build 'em faster. Also, with unique coverage, Shy'dow ships take less hull damage (10% less damage), making them rather durable. Lastly, you want to go to war, because you will earn massive amounts of salvage from the ships you kill, and each successful battle will allow you to significantly expand your fleet. Shy'dow have a weakness, I just don't know what it is. Seriously: Shy'dow have no inherent military advantages, and few species have good hab planets for them to consume.
Sirnef
Style: Builder, Expansionist, Flexible.
Difficulty: Medium.
Comment: Sirnef have no military or economic advantages, and have an atrocious population growth rate. They have an average starting economy, but it is very difficult to develop the economy because of the slow population growth rate. The strength of the Sirnef is derived from the fact that just about anything is good hab for Sirnef, and they have fantastically efficient research capabilities. It takes discipline and forethought to play Sirnef. The cheap research is not a universal advantage, it is best exploited if you focus on specific topics (economic or military) to specialise in. Also, the good habs all round means your best chance at success is rapid expansion, which requires a very disciplined and well-managed population growth strategy, which has to be maintained throughout the game. A secondary weakness is a player perception that Sirnef are useless to an alliance, which is not the case. An active alliance and a generous Sirnef player can make an excellent planet-swapping combination, with no-one having to colonise bad habs. A sirnef can accept the worst planets other players have to throw away and it will be good hab for him, while at the same time having few reservations about giving away 80 hab planets (or better) because of the sheer wealth of good habs available to the Sirnef player.
Toag
Style: Builder, Expansionist.
Difficulty: Medium to Easy.
Comment: Toags have a medium strength economy to start with, bolstered by a superb credit income. This makes them a fast expanding player early on, and with the unique later on a Toag will have the fastest developing colonies in the game, and nothing holding them back from rapid expansion. Toag ships have cheap sensors, but otherwise no military bonuses. Most species are good or medium hab for Toags, giving them lots of options for rapid expansion.
Turian
Style: Expansionist, Militaristic, Flexible.
Difficulty: Medium to Hard.
Comment: Turians have the second worst economy at game start (Gorgoth are the worst). However, they have good habs all-round, and are the second best researchers in the game (after Sirnef). Turian ships are the most durable ships in the game due to their massive shielding. If you can sustain the population growth, Turians can expand at a prodigous rate through just about any species. On the defensive, a well-shielded Turian planet can hold up huge fleets for many ticks, giving the Turian player and his alliance time to rebuild or re-organise.
Zeeb
Style: Builder, Militaristic.
Difficulty: Easy.
Comment: Zeebs have one of the strongest economies from the start of the game, and can build ships faster than any other species. This makes them a powerhouse in the early stages of the game, because they can rapidly convert their economic advantage into military strength. A good start to the game can ensure the Zeeb remains a powerhouse throughout. On the defensive, a Zeeb can recover faster than any other player, rapidly laying gown space-stations and quickly rebuilding lost ships. Another advantage is that a zeeb with good research can convert that tech advantage into real ships faster than any other player.