One big diffrence here...
In the GBA the 8-bit GB processor is inacessable in GBA mode(total waste of a chip, and evidence to support my belief that the GBA was rushed to market to stop Bandai and SNK).
In the DS both CPUs operate in parallel in DS mode. And the performance disparity is small enough that the system can really be considered a dual-proc device.
Anyways... I'll trade my GBA in for a GBColor when I get a DS.
Neither incarnation of the GBA is really pleasing to me(one's got a crappy d-pad, the other's too damn small). The DS has neither unit's flaws(well, not untill the LiIon packs start failing...). And while teh GBC isn't my favorite system ever, it's the most convenient 8Bit GB.
you will need an sp should you want to play gba games with others or hook stuff up to the gamecube.