IMO, get whatever suits your gaming needs. I have an SP and a DS(well "had" a DS, waiting till the next shipment of DS Lites arrives at EB so i can pick up mine), and i usually only use my SP to trade to and from my other pokemon games on the GBA. I bought the DS espescially for Pokemon DP, but seeing as im gonna have to wait a while, im buying some others in the process.
Now, if you want a DS, go right ahead, they're much more affordable now than they were 2 years ago(duh, that's the usual time frame for most consoles to go down price). Anyhow, If you still want to trade between your pokemon games, then id suggest getting a GBA/SP/Micro. IMO, The SP is the better of them, but if you want comfort go for the GBA.
Like i siad, go with what you need.
-C_L-