I say Dialga. Reasons:
1. The thing looks like, 5000 times cooler.
2. People will often say that Palkia has the better speed stat, but Dialga has better special attack.
3. Special move. When it comes right down to it, when a Dialga and a Palkia of the same level battle using their exclusive moves, Dialga would win. Sure Dialga has to rest after using Roar of Time, but thats because it's powerful. Plus, Roar of Time is super effective against Palkia, and Spacial Rend is only regularly effective against Dialga.
