Really, they look like some sort of undeads from that-other-mmo-game. Pale skin, empty grey eyes... Can't unsee.
But seriously, I think they'd make best heallers considering the Superior Pacifist trait, in addition to Project Image (threat reduction).
Still, on one character I've made a sci boff, while the second got a tac. Just for fun - I don't think it really matters in PvE that much, even on Elite difficulty. The question would be more valid if there was an option to create a playable Aenar character.
Catian for tactical - all their passives make them great ground tactical officers.
Aenar make great medics. Their bonus to kit healing powers are awesome and their project image passive skill decoys agro away from them, keeping them alive longer to heal.
One of my toons has a Caitian TAC, who does an awesome job. If you train a Caitian TAC boff in Lunge, they'll get *two* good hand-to-hand attacks; the Lunge and the Caitian Pounce. They also have nice passives for combat.
Another of my toons has an Aenar medic, who also does a great job. When I use rally points to set up my boff ambush and I go ahead to start the attack, I know the Aenar's Projected Image will be running up alongside me nice and quick. I've seen enemy mobs pay more attention to that image than to my toon who's doing ACTUAL damage to them. Another combination I've tried is to one-shot a mob member, allow the image to run up and then I toss in a stun grenade. At that point, I let the boffs free from their rally points. I call this the "Release The Hounds" tactic.