Most people pick Diamond. Go with Pearl if you want to be original. Diamond has cooler pokemon though so if you don't have wireless internet then go with that.
EDIT:
I've seen 260 and owned 163 (Just caught Giratina). My proudest accomplishment though is that I got a level 100 Parukia (since it's Japanese I use it's Japanese name) for a level 9 Shieldon. It was awesome.
I caught Cresselia, Giratina, Mesprit, and Heatran today.
I was exploring a cave today and came across Giratina. I'm absolutely shock how easy it was the capture it. It was easier than capturing a Slakoth with it's legs and arms cut off. :lol:
I wont be ready for battles for a while until i get my party rounded out and such, but for now there it is. I'll go ahead and add folks from here soon ^_^
Once i can get a hold of a friend's DS to use, I'm going to send the egg from Ranger to my game and get a Phione. I can then breed it to get a bunch of manaphys and I'll be trading those for things later. I wont trade the Phione though, can only get one of those :P.
You get 1 manaphy and it breed phiones. Phiones are decent bargaining chips though. And since I had to clear some rubbish out of my boxes I'll give anyone who needs them a few starters and rareish pokemon.
If you have a Roserade then I'll trade you for it. You can keep the ditto and I'll send the Roserade back. I just want my Sinnoh Dex completed (as in all 150 owned) and I'd much rather use my dawn stone on a Gallade then a Roserade that I'm gonna shove into a box and never use.