Title should be:
As what faction do you usually play?
;)
and humans, because they are significantly more pwn.
On the whole, humans are used to dying - dragoon headshots left right and centre give them a generally poor lifespan. When they do retire to their early grave it's generally with a sigh of "oh no not again".
When, on the other hand, an alien goes from 200 hp to 0 because I just emptied a rifle clip and a half into him, he's much more likely to be all, "wtf mate :(" and I'll be like "lololol". Much more rewarding.