I own and like both games but I'd have to give it Radiant Dawn. VC is definitely the more innovative of the two but I beat Radiant Dawn 3 times and have put close to if not more than 200 hours into it. As soon as I beat Radiant Dawn I started a new quest and thought about the different units I'd use and I did the same thing following the my second time completing the game.
As good as VC is, it's one of those one and done games. You beat it once and there is little reason to play through it again. I played VC on my roomate's PS3 and after he moved out. I bought my own PS3 and VC. I'm only up to chapter 9 on my PS3 because it doesn't have the same draw the second time through. I had to stop myself from playing Radiant Dawn because it was monopolizing time I could be using to play my other games.
