The iPhone is a great phone. I'm sure you will like it. Now that I said that, here are my thoughts on your points.
1)The guys at the office that went iPhone to Android always speak about how much better the large screen real estate is. I went from an old Droid (iphone size screen) to this and I love the extra real estate. I am more prone to view full web sites and not the mobile versions. The size really turns out to be great. It fits well in my pocket and the visibility in the car dock is awesome.
2)Android app sharing is available on all devices. I have all my apps on my phone, my old phone, and my kindle fire.
3)Backup. iPhone does a better job. Android backs up settings and apps but does not back up app data like how far you have gone in Angry Birds. Android now supports that capability but few apps have taken advantage of it yet. All my apps, settings, and wi-fi access points all were automatically installed when I logged into my Galaxy Nexus for the first time.
4)iPhone does have a deeper app market. One thing I noted it that the apps appear to be more expensive than the apps I find on the android store. It may just be that there are more paid apps that give this perception. We stopped using our iPod touch because all the apps we wanted were a few bucks and we found comparable ones on the android market for free or cheap (~99cents).
Either way I think you will be happy. Both have positives and negatives. I would recommend you try out the Galaxy Nexus. The new Ice Cream Sandwich OS is so polished now. It finally feels like strong competition for the intuitive user interface of the iOS.