Depends how you use it; if you're one of those people who gets stuff gets covered in scratches get an SP for the extra screen protection (I know you can replace faceplates on the Micro but they are somewhat hard to get hold of), otherwise go for the Micro.
Personally I've never liked the SP, but it comes down to how you use it as to which one is better for you. I love the Micro; so small, so shiney and stops me looking like a total idiot on the bus.
The shoulder buttons aren't as hard to use as everyone makes out, and the screen size isn't a problem at all.
[Edit] Oh, and the Micro's probably cheaper if you know where to look