I'm gonna go for MVP on this. Before I say why, I just want to make it clear, that at this point neither of them are getting a main event push anytime soon. MVP is finally winning again matches again and is gonna need a solid mid-card push before we could ever image him with a world title, and Kennedy, he's still out with an injury, and can't seem to stay healthy.
Anyway, I say MVP for this reason, he just comes off as more legit for me. In 2007 Kennedy won Money in the bank, so it WWE obviously had faith in Kennedy's main event potential, but then he got injured and came back for a little while before being caught with steroids so he lost all momentum from his MITB win. Kennedy has the mic ability which takes you far in the WWE, he's also way over with the crowd even when he was heel. I don't know why people diss his in-ring ability as it is where it needs to be. His matches with Jeff Hardy (both on RAW and on PPV) were good, and his match with Matt Hardy a year or two ago on SD was enjoyable. But because of his being injury prone the WWE can't trust putting a big title on him.
MVP has only been in the WWE for what like 2 years or so now, and he's already the longest reigning US champion in the company's history. He had numerous good matches with Chris Benoit (not afraid to say his name), his feud with Matt Hardy was good, and is great on the mic. When need be he can really get heel heat from the crowd, and so far seems not to be as injury prone as Kennedy.