I got the white mac (new, not refurb or used) instead of the alu, because:
- Cheaper: Mine was more expensive than the base Alu 20", but I didn't want a 20" screen (especially after hearing that they are terrible in quality). The price I paid for the white 24" was a good deal cheaper than the entry 24" alu (let's just say more than $60 difference!)
- Matte screen: I prefer it
- Lots of 'issues' reported with the Alu's: They seemed to have died down a bit recently, though I haven't been really following. I just didn't want to worry about potential freezing/gradient/condensation problems.
- Styling, I consider it a wash. I do like the Alu idea, it's just that the combination with the black I'm not 100% fond of. And the new plastic black backs (which I've seen oooh'd and ahhh'd here on more than one occasion

) even remind me of PC's. *shudder* I think it's also more inconsistant on the whole too, once you factor in the white mighty mouse and the white keys on the keyboard. Heck, the new keyboard looks more at home with a white mac than the alu.
Happy shopping. I'm fairly sure you'll love either Mac.