Ok without going into too much detail, I've tried all three and my findings are to get the best emulator experience (not to mention overall tablet experience) if you can afford it, go with a true windows tablet. Buyer beware though... there are windows tablets and there are windows rt tablets. Windows rt is kind of like windows ce back in the day. It's windows "ish" and thus not all programs will run on it. If you get a windows tablet though, you essentially have a laptop. Windows 10 is the first windows OS that fully supports tablet features, so you are good to go. Gamepads just work as you are running windows (DUH).
Apple tablets are crippled by the fact that they have to run the awful iOS. If you want to use a gamepad or what have you, be prepared to fork over more money for a Bluetooth gamepad that is compatible with apple apps.
Android tablets are fantastic.... when you get an expensive one. If you get an expensive one though, it isn't much more to get a surface or an asus convertible. You can get gamepads and other USB devices to work on some of the higher end tablets, but if the actual app supports it is a crap shot. Thankfully most emulators do.