I own a first gen iPad Mini and love it. I had borrowed a friend's iPad 3 to see what I thought, but after using it I decided that although the larger screen is nice, it's just too large and heavy to be used casually. By that I mean (speaking for myself) it's too heavy to conveniently hold for a long period of time with one hand. Sure I can kind of balance it with one hand, but eventually my wrist gets tired and it's just a pain. My iPad Mini, however, is just the right size. Physically I can hold it in one hand, griping with my thumb on one side and my other fingers on the other.
To me, my first gen Mini's screen is fine. Sure if I compare it side by side to a Retina display it's not going to look at sharp, but looking at it alone (for reading, browsing, games, etc.) it is perfectly fine.
As for the screen size, I think once you have it you'll be fine with it. Your mind is going to say that you want the largest one, but honestly in terms of actual size and weight, I'm perfectly content with my Mini. Especially the fact that it's super light and very thin.
And as for the OS (Apple/Android), I think it all depends on your other devices. E.g.: I have an iPhone and Macbook Pro. Therefore I was able to have many seamless features between all three devices since they are all Apple products. If you don't have any Apple products, I would probably recommend looking towards an Android device. Although I don't know much about the tablet market anymore, I would recommend the Google Nexus 10, as my girlfriend's brother has one and loves it.
Hopefully this helps.