Well, I don't own any of the guns but I've tested them before and been in combat in spar against them.
AB8 - I like calling it the mini Souzien. Fast gun, fast reload, hardly any freeze, clip size of 16, fires straight bullets (100% accuracy), automatic, 15 changeable camos. I would personally say its a very good gun to buy mainly because of its very low freeze when fired which I can pretty much assure is lower than the M4. It is a deadly gun in spar and it doesn't have any drawbacks APART from the fact it's still af 8 damage even when it's at 'perfect condition'.
Don't know much about the M8A1, sorry.