I installed the Afterburner kit on my GBA (white) and it was a bitch. But, I think the GBA+afterburner > GBA SP. The SP is too close together and gives my big fat hands carpal tunnel. Also, the SP is like a complete rip-off of the Afterburner mod considering anyone that modded their GBA with Afterburner can tell that the SP's screen has the same 30degree angle reflector material as the afterburner kit.
I've since sold my GBA.. But if I were gonna get a kid a GBA or SP, I would get the GBA + Afterburner kit. The SP's buttons are just way too close together.
Also, battery life isn't much better on an SP considering you can use normal NiMH rechargeables in it and you can get them at 2200mah now. I got ~8 hours of life with my Afterburner on full brightness (never installed the dimmer) with 1600mah NiMH batteries.