I'm not really a fan of either ergonomically since I have big hands.
SP would be the better buy though. Cheaper, better screen (assuming you get the AGS-101 backlit model), and compatible with all Game Boy games.
How does the SP have a better screen than a Micro?
I'm just curious.
The Micro would have the better pixel density.
I personally have huge hands and the Micro ergonomics aren't too bad, I bend my pointer fingers to hit the R and L buttons, and my thumbs to hit the D-Pad and A/B buttons.