When I typed "It depends on a few matters: The online stability; the gameplay; and which characters of the roster I like (as characters, not fighters)" I mean it depends on all three; not putting all the weight just on the online, though I did list that before the others.
If it turns out the online isn't much better than MK9's online, then it'll increase the amount of times I'll play MK9 again over GAU; even more so if the gameplay doesn't fit my style and/or most, if not all, of the characters aren't so relatable to me.