Its more portable - stupid I know for a tablet - but the Nexus is easier and better in the hand. Personally - I prefer the screen to the iPads - although the colour saturation on my S3 is much better than the Nexus 7. It feels lighter - and at 7" is pocketable (just)
Obviously stuff like the OS is a preference thing - but the integration with my S3 is brilliant - especially the syncing with Chrome and googlebooks.
When you consider a 32gb ipad2 is almost double the price for a poorer processor - less (IMO) attractive screen, no widgets, half-arsed notifications support and the iTunes tie in ..... its a no brainer.
Oh and you get a free movie. (I got Ice Age) - not great but a nice touch when you iTunes just give you a bill it help.
I'm sure our more technically minded friends could give you a far better arguement but thats my two cents.