I have to take the context of time into account and vote for the SP. Everyone had wanted a lighted screen since the days of the original Game Boy, when we had the Game Gear (and even the dang Lynx) to compare to -- after the Pocket, the Color and the Advance all failed to address that, finally answering that ridiculously overdue feature just made the SP feel like the ultimate Game Boy at the time. The industrial design was slick, the clamshell was relevatory, and it could still play the full range of the Game Boy library. Not having a headphone jack could be seen as an unforgivable sin (almost as unforgivably sinful as the dogshit audio quality of the GBA itself), but the adapter is a fairly easy solution (and, strangely, one that we're now begrudgingly used to with many 3.5 mm jack-less phones).
The Micro is a thing of true beauty, though, especially the Famicom edition. One of the biggest design swings Nintendo has ever taken, and I adore it for that.