I'd say go for a DS Lite. It is backward compatible with GBA games, the back-light is far superior to SP and of course you have the option of all DS games.
But if you're not interested in that idea, then SP is the better option. It has backward compatibility for the older Gameboy/Color games.
The GBA library in general is great, you just have to find them these days since most stores don't sell them anymore (mostly family-owed business and online stores). It was a worthwhile generation of games.