I know this is off topic but let's get one thing straight. Gpotato has very very crappy servers. Gpotato is based (and their servers are too) in San Jose california which is 2 hours away from me and I often got some bad lag from their servers. Oh yeah and I'm on cable.
Corum and FlyFF are both ok games, the only thing is that Flyff is crowded and has less of a point ... and both are just click /auto attack type games...which i find really boring >_<
So I'd say neither, id say go with something that involves skill and level. A balance.
But if your heart is set on one of the two i would say Flyff because the comunity is better, is easyer to understand, etc...
But an exact replica of Flyff is Scions of Fate (Yulgang) Its basically Flyff with better clases, better graphics, and entertains more.
Corum lags like ***** with a hernia when your in town. But, when your in dungeons an whats not theres like non at all :P. Flyff is still funner.
and scions of fate isnt an exact replica. Sof is a martial arts fantasy while flyff is like cute-medieval. flyff has more customization, you can fly in flyff (:P) all the mobs in flyff arent aggro( >_>), and more people play flyff and some more stuff that I dont feel like saying. So imho, flyff is better.