The relative quality of two huge app stores is tough to guage, but I think John Gruber made a good point when he said that there's a shortage of exclusive Android apps that Apple would allow in the iPhone App Store. Most of the highest-ranked paid Android applications of 2010, according to Distimo, are task killers, widgets, rooting tools, and backup utilities. Looking at my colleague JR Raphael's lists of must-have
Android apps, those that don't fall into the "not possible on iOS" category are either made by Google or already available on the iPhone.
Meanwhile, numerous must-have iPhone apps aren't available on Android. And some apps that exist on both platforms lack certain features in the Android version. Paypal, for example, can cash checks on the iPhone but not on Android.
Advantage: iPhone App Store