Either work really. It's easier to do multitouch buttons with virtual keys, but GM:S can handle 5 mouse click events at once as I recall. Should be more than enough as people generally only use two touches at once.
Im wondering if this has changed over the last year, as I have a 200MB game I with lots of sounds and music that I would like to upload as is. I could maybe get down to 100 or 150 without the music, but I don't think I can push 50 without severely changing my product.
Rather old Necro post. It is now possible to split your APK within GameMaker studio so that you have the 50 MB installer and additional content files. You will have to look into the documentation on how to do this though.