Ok, so there are technically 5 units in question:
1) Original GBA SP
2) Updated GBA SP with the newer screen
3) Nintendo DS
4) Nintendo DS Light
5) Micro GBA
I have witnessed first hand (1) (3) and (5). I'm going to guess the above is also the right ranking of screen quality, from "worst" to "best". Although, (2) and (3) may be the same screen. The micro's screen is actually quite impressive... reminds me of the PSP in the luminance and clarity. Is there any word on what kind of screens will be in the DS Light? I'm guessing it will use the same setup as the micro for volume and brightness adjustments (for those who don't know, it uses up and down push buttons for volume, which also double as a brightness adjustment if the L trigger is held). That doesn't mean its going to look like the micro in quality though...
Micro really is a *sharp* screen. To bad the units don't lend themselves well as in store display units, being small and hard to secure...