The MP500 seems like a bumped-up version of the MP450:

Screen: 2.1 vs. 1.9 inches
Accuracy: Small as 1 picoliter vs. 2 picoliters
Time to print photo-quality 4X6: 51 seconds vs. 55 seconds

I would choose the MP450 for $50 more dollars and it has Bluetooth, infrared, and PictBridge.

Just my $.02

~Ibrahim~

P.S.I would go to a store that retails both, try them out, ask the salesman/saleswoman if they'd let you test print one of your pictures.