The iPad 2 will likely ship with iOS 4.2 or perhaps 4.3. Any advantages will have to be hardware based, cameras, faster, more ram, thinner, etc., and perhaps App based, iMovie, iPad optimized FaceTime, etc. It will be interesting to see if an iOS 5 update is coming soon, and if so will it be previewed? I would think there will be some major enhancements to iOS 5, and possibly some divergence between iPhone and iPad versions. We could then compare all the previews and declare which is superior without ever touching an actual product!
It'll suck if the iPad2 has to wait until November to achieve parity with the Summer 2011 iPhone.