I haven't used the Windows tablet app store, but I think Android has a lot more refined and useful widgets/apps (especially widgets, I'm guessing).
Windows and all its programs are fine. But I like my widgets in Android for a tablet.
And there's the fact that 12" is much too large for a tablet, and on the very small side for a laptop. I just want to do basic (but more advanced than current) Office tasks/work on a MOBILE (read: 8-9") tablet. I think an Android tablet can potentially "upbridge" the phone/laptop gap more than the SP3 can "downbridge" that gap.
Well, when you are on true Windows and can run any real app under the sun, the need for an app store is a bit less, I think.
There is very little that an Android tablet can do that an Android phone can't. It is essentially just a large phone, without the ability to make calls...