I've got them both.

I'd say that motion flex v2.1 is better overall. When they say "Cobra is laggy", it's the menu itself that lags and it kinda does. No major problem with Cobra (yet).

Motion flex has got many bugs, especially with the gametypes. Cobra has got much nicer game types that run far more smoothly without error. That doesn't mean Cobra is flawless, for some reason, when you activate god mode, as soon as you use something else on the menu, you lose god mode... I'm not 100% sure (yet) but, I am unable to play with Cobra unless host (this is kinda small since you're likely to be host with mods - people would nag you to be host :/).

Cobra has got a much nicer infectable menu, Motion flex is pretty basic.

Cobra has also introduced different and more up-to-date mods.

However, Cobra can only be used on Der Riese and Der Riese only, where as, Motion flex is able to be used on everything.

Motion Flex is better.