Platinum is superior to Diamond/Pearl in every way, just to note. More pokemon to catch (including legendaries), much more wi-fi capacity, and the mechanics are fine tuned. Gameplay is also sped up without needless intervals, so you don't have to wait a second just for your attack animation to start, etc. The graphics also seem slightly more trim, IMO.
It comes out in Europe May 22nd as well, correct? That's not too far off, yeah?