Android is (mostly) unchecked and unsecure. Recently Google has been forced to do something about this problem, but there is still plenty of malware for Android.
The iPhone App Store, being curated by Apple, doesn't really have this problem at all. That's not to say you can't get a low-quality, poorly-made app -- but Apple does tend to check that it doesn't abuse user info, does what it says it will do (is not scamware or a trojan) and so forth.
At this particular point in time I can think of no reason why anyone would purchase a non-Apple tablet except:
1. They have more money than sense.
2. They have the money to blow on trying out something different.
3. They enjoy half-baked, half-finished, insecure software with no visible upgrade path.
4. Can't stand Apple.
5. Prefers a 7-inch screen you have to hold a lot closer to you to use.
I'm sure Android for tablets will continue to improve at a sharp clip, but *at the moment* someone would have to be pretty well nuts to invest in an Android tablet from a value perspective.