Well right now on the DS they have a few different Poke'Mon games.
Diamond & Pearl, and the soon too come Platinum.
These are the games most people play. The thing about them is simple... They are all the same besides the pokemon on the cover. They hold a few different things each but all three games are the same.
Then there are
Pokémon Mystery Dungeon: Blue Rescue Team
and
Pokémon Mystery Dungeon: Red Rescue Team
as well as...
Pokémon Mystery Dungeon: Explorers of Time
and
Pokémon Mystery Dungeon: Explorers of Darkness
and!!! (they just won't let it die...)
Pokémon Ranger
as well as...
Pokémon Ranger: Shadows of Almia.
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So to answer your question on which to get. If you have played the older Pokémon and want to the same game play go with Platinum when it comes out.
If you want to try a different type of game play then pick one of the other games listed. They are different from the older games
Hope I was able to help you spend some money.
-TwisT