I usually don't offer comments on what to buy because I feel this is REALLY extremity personal thing, just like a buying a car one can thing and have some priorities other may have totally different things that he or she may like. With that said I'd like to get my side on your question.
1. With your budget you can almost get any tablet you want anywhere from iPad to Android and Windows 8 units.
2. With amount of programs available for all systems I must say that yes iOS have more quantity of programs but I must reassure you that most, no ALL major programs that people use available for iOS and Android, and I really mean it. If you really want just check out iTunes store and Play store and see for your self, all major games , apps, movies available for both systems. Windows is a bit different and I'll not mention it yet because I feel they are not there yet. IF you want Windows 8 Tablet , get a laptop or something. So I hope I am clear on software, quantity yes but quality not so much, both have all software you may ever need. And let's say if one have a weeks early , trust me another will have it as well.
3. Performance. You'll get exactly what you paid for and because technology advance so quick I have to say that you no longer need to get iPad 4 to play cool games. Few Days ago New Nexus 7 announced and you can get one in few days that will play anything you want with new OpenGL SE 3 that really move pixels fast. So GPU is no problem on any of the system, and I must say because new Nexus 7 is latest product it kinda faster than anything else in that size. Now operating system itself. iOS is really smooth and designed to work with apple stuff, when android designed just to work and I really prefer android here because I am not forced to use iTunes if I want to access my device, so freedom of such is important for me. I've been using Android and Apple for years now and I must say that Android and all recent devices are really smooth as well and I don't think this is an issue anymore because devices are so fast that you simply wont notice anything. So performance wise I must say latest is better and because Apple product is several months old and new Nexus 7 just came out it's faster.
4. Screen size. with that I have to ask you just go to store and play, for me iPad 3 and 4 were just fine, but 7' units I fell in love with, so look for your self.
5. MONEY. Nexus 7 16GB version $229, Apple iPad mini $329 and iPAd 4 $499
So here it is.
And finally I want to repeat myself and say that this is really personal thing, I am a geel and consider iOS for more no0bs device when android can do more , I can mess with it, can install custom roms and just more phun. But if you need one for work or need super stability and stuff get iOS. But again Android just as good if not better. Choice for yourself and don't forget one thing, no matter what you get you'll enjoy it cause this devices are Magical. (SJ RIP)
My last work, look in to New Nexus 7, I ordered mine just few minus ago, it's amazing.
Good luck !
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