for the app? if so, ill likely start with iOs as i currently do not own any android devices. Will likely optimize for the iPad initially - because big screens. I also have more experience developing for iOs than android.
access tuner only works on windows, which is sad.. but asking for competition.
for the car? i love the subaru, but ultimately would like to get back to porsche.
1. I would like to see a platform supported that doesn't cost Apple money nor locks you down to an iOS. At that point, people can get a cheap Windows laptop and press, which is what I used to do.
If you're looking to tune from this device, it's going to start getting more difficult as you'll want a keyboard... Generally a mouse too, as when you try to fit more on the screen, my fingers are too fat to be exact on selecting items.
2. Android is the way to go, especially with how cheap you can get an Android device and that you can put Android radios into cars easily. Hell, I'd prefer software for a Chromebook over an Apple device (yes, I have a work iPhone... Haven't turned it on in months).
3. I guess that really depends how far you're trying to go with a tablet platform. Utilizing drop-downs for know product mods to add/mix in aiding in tuning, and shortcut buttons for shaping things that are specific to that car.
4. Depends on the usability, accessibility, portability, and what else is out there at x price point.