THe GBA SP is the same hardware internally as the GBA. It is shaped like a flip-out compact style device, and has a lit screen for playing in poor light. The biggest non-cosmetic difference is that it has a lithium ion rechargable battery rather than two AAs. This of course creates a serious problem if you lose the recharger.
If you intend to get a DS, then I don't think getting an SP is worth it UNLESS you want to continue to play old GB and GB Color games, because the DS is backwards compatible with only the GBA.