Why sinnoh? Before you say it was slow, platinum fixed that, and added a while bunch of stuff.
I haven't even played D/P, and I can still say no it didn't. Platinum still runs wicked slow. If D/P were slower, then...ugh. Platinum might be quicker, but it's a far cry from fixed.
But I'm still picking Sinnoh, because I love the region, and Platinum is still a great game. It's got a good variety of locations, and a game path I've really come to enjoy.
I have a hard time picking a single Pokemon from the region as a favourite. There's a lot I like - I am a fan of the new evolutions to older Pokemon thing. I guess if I'm picking an all new one, it's got to be Torterra or Drifblim.
I like Unova as a region, but I am not a fan of the linear game path through it. The post game being all over the place is nice.