I am in no way biased, but play cryx.
They have evil undead creatures and horny women. they even have undead pirates. what is not too like....
real reason: if you like a army with a lot off fodder, troops that go boom and have lots of sneaky ways to go for the kill then cryx will be your cup of tea. they are fast and deadly, but have low armour because of it....
retribution I can't really comment on, as I do not have them and have not played against them yet...
good luck picking a side (as long as it is cryx

)