The different types of GBA have traditionally gotten a bad rap because wave playback kind of sucked. However, now there's a fix for that makes it sounds not completely terrible. (Thanks self for discovering this!)
I know this wasn't in the question, but avoid the original GBA. The button layout sucks for LSDj and there are no good and widely available options for backlighting it. (actually frontlighting, technically speaking.)
There are two models of GBA SP, and one has less background noise than the other. Other people will know more about that. And as mentioned above, GBA SP is a small neat device, though original GB cartridges will stick out a fair bit, which takes away some of the neatness, if you want to carry it in your pocket, say.