It's a difficult question to answer because both these guys are damn good. I'm sorry but you just have to be an absolutely closed minded fool or complete WWE hater to not be able to see that both these guys are stars and are going to be in the main event picture at some point in the future.
Both Ziggler & Rhodes have worked hard to get their current spots. Cody Rhodes has just consistently and steadily improved since the split of Legacy. When he & DiBiase faced Randy Orton at WM 26, they both still seemed like scrubs. Since that time, Rhodes has come into his own and has improved in every relevant area where a wrestler might need it. He grew as a character, grew in mic skills, learned how to tell a story inside the ring and has been easily the best IC champ of 2011. His feud with Randy Orton really, really increased his stock. He did have one victory over Orton but he still came out on the losing end. He did ultimately, however, come out looking like a star and his feud with Orton did do good things for the IC title. As I've said in other posts, when's the last time a guy who was a World Champion at this time 5 months ago and a 9 time World Champion overall feuded with a reigning mid-card champ?
When it comes to Dolph Ziggler, Ziggler is someone that's scrapped and clawed for every inch of ground he's gained in WWE. Ziggler was always a solid performer but I really started to become a fan of him after he won the IC title. He was an ideal IC champ that defended the title a lot, had memorable matches, interesting feuds, elevated the status of the title and was thereby elevated because of the title. In 2010 and 2011, Ziggler has been in some incredible matches where he's just flat out looked like a star against John Cena, Randy Orton, Edge, Daniel Bryan and CM Punk. The only thing missing from Ziggler was that we didn't really know much on him as a character or on the mic. The WWE has used Ziggler's time as United States Champion to build a presence for his on-screen character. He's gotten some real mic time, he's dramatically improved as a character, he carries himself like a star and has become quite solid on the mic where I think he can only improve as time goes on. Ziggler has had the goods inside the ring, I think that's been pretty obvious for a long time. Now that he's developed and continues to develop his character and his mic skills, I can't see him going anywhere but up.
In my view, no matter which guy you choose, I don't think you're really gonna go wrong. Personally, I'm giving the edge to Ziggler. He shines inside the ring and I think if he ever gets some lengthy promo time, he'll be able to shine on the mic.