Aliens:
s1: has goons, which pretty much beat all s1 hummies unless they're good with shotties/chainguns
s2: has adv maras, which are good, but nothing compared to the flamer, prifle and helmet the hummies have
s3: tyrants and adv goons, basically the aliens' best stage. pretty much equal to s3 hummie upgrades
imo I think aliens should get something else at s2 to make it a stage that's worth something (right now, I only notice it when I'm a builder or when I specifically want to be adv mara), and hummies should have maybe slightly cheaper chainguns to combat goons.