A big point of difference to note is that the M!x is the first with the mixing feature, and much is the same in the Meets/On including characters and other features. However, the Meets/On have the bluetooth feature that lets you bring your tama into the official app to play games and marry other peoples tamas, as well as download items, backgrounds, and gotchipoints.
If your fear is that the bluetooth feature will be useless if the official (phone) app eventually closes down, don't worry! I assume with the unofficial Mymeets (computer) app you will get long term similar playability. It works with bluetooth the same way, and lets you creates custom characters to marry, plus it has compiled a selection of all the downloads available straight from the English and Japanese official apps. It's useful as a cheat program but doesn't have to be.