Throw away your copy of Diamond and Pearl.
You don't need them anymore...well maybe for the exclusives in those versions but that is the extent of your need. Platinum is a much needed fix on the games, improving an incredibly slow user interface that made an average Pokemon Battle last 5 to 10 minutes instead of a minute and under. The difference in that speed is truly mind boggling and more so that Nintendo let the slow user interface of Diamond and Pearl release into the wild.
Other changes present involve the battle tower like the last two 'third' editions to release (Crystal and Emerald), some new move tutors, new events involving the 'PokeGods' (See Gimmicky Nintendo Events at an out of the middle of nowhere area near you!), and a new intro.
The new intro actually changes the beginning dynamic of how you get your Pokemon and puts more emphasis on Team Galactic.
Verdict: BUY