Ahhh man, I might sound like a total dork here, but both of them are great. I think that Little Big Planet is really entertaining when playing in multiplayer, but I have never played it by myself. Viva Pinata on the other hand is a great game to just sit down and play for a few hours by yourself. It can get a little tedious though, getting all of the different evolutions and breeding each pinata so many times.
Unless you are a child, I would not recommend Viva Pinata. LBP is just... epic, in so many ways. I've never played Viva Pinata, but I imagine it's one of those younger child games.