I want to pick Kofi, I want to pick his so bad. He's one of my favorite wrestlers right now and he has all the tools to make it big.
That said, I think Cody Rhodes is the one who's going to make it big.
Cody Rhodes has a lot of things going for him, starting with a great gimmick. Kofi has rarely been given the chance to show personality (and when he does it's a smashing success), so it's hard to say what his gimmick/character is exactly. Cody Rhodes is easy, he's an arrogant narcissist who cares more about looking good than winning, though he'd gladly take both.
Wrestling wise I give the edge to Kofi, but that doesn't mean Cody is a slouch. Cody Rhodes has gotten much better since his Legacy days, and has really carved out a style for himself. Kofi is definitely much more unique than Cody, and his style definitely stands out much more than most of the WWE roster, let alone Cody Rhodes.
When it comes to charisma, I'd say they break about even. Kofi interacts with the crowd while wrestling more than any other wrestler as far as I can tell. Between his boom boom claps, his punches in the corner, and his fast paced offense, it seems like he does a lot of stuff for the sake of the crowd. But then again, so does Cody. There's few things more amusing than Cody Rhodes interrupting a match to make sure his face isn't damaged, and he really has the audience eating out of the palm of his hand when he uses those antics.
The reason that I think Cody is going to go farther than Kofi is because of his pedigree. He's the son of Dusty freakin Rhodes. That's right, it's not like his dad is IRS, his dad is one of the most legendary wrestlers to ever lace up a pair of boots. Whether or not that makes him any better a wrestler is irrelevant because fans will always see him as Dusty's boy, and that's a big draw. Furthermore, Kofi's last high profile feud was with Randy Orton back in 2009. That's a long fucking time ago. Meanwhile, Cody is about to enter a program with Rey Mysterio, and Cody will probably go over Rey due to Rey's injuries.
Like I said, I wish I could pick Kofi here, but all signs point to Cody being the superstar of the two. Kofi's talents won't be pronounced enough if the WWE doesn't give him a solid push, and they haven't since his feud with Orton. I hope they do, but until then, it looks like Cody is going to be the more successful one.