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.
I personally would go for the DS Lite, but if you still have RBY or anything else from those days, and you just might want to go back and play them, get a GBASP. If you don't have those games anymore, get the DS. Either way, you should still get Sapphire. If you get a DS and suddenly want to play RBY again, but don't have the cash for a GBASP anymore, (unlikely), get Fire Red or Leaf Green. You probably know this, but they are enhanced remakes of Red and Blue.
By the way, the DS has a better backlight than the SP, and the DS Lite has an infinitley better backlight than the normal DS.
I prefer the DS myself. Though I do own a GBA too, I bought the GBA secondhand and whoever last had it didn't let the battery recharge properly... The battery is supposed to last 10+ hours, mine lasts 7+... My biggest hope is to find a replacement battery...
I advies DS aswell.DS cant play colours unfortunately but there arent many good colours so I say get DS as it is also handy for Advance
And besides,Diamond and Pearl are coming out soon and you cant play them on a GBA.