For me it is:
You could make it more complicated but, I think this is simple enough. If it's not enjoyable you will probably stop doing it. Ideally there is some money to be made in it, or at least that can be used a sorting order. If both are fun, pick the one with the most cash or sale potential.
I wish I wouldn't get distracted with side projects but I do. I can't help it and now I embrace it, but, like you, i want to get better at setting aside time and limiting them to only a few so they can actually be beneficial.
There have been no amazing successes so far. (well, not financially. Technically almost all have been successful - and that is the old cliche/story... no paths beaten to these doors just yet)
Is it necessary to have a side project? Is it possible to find a main project so exciting and with so much potential that there is little reason to work on a second project? My advice is to prune all side projects and to focus entirely on a project that you absolutely love.
Is the project going to aid in the realization of your long term objectives?
My criteria is relevance. Does this project have any relevance to achieving my single biggest goal; is it helping me gain relevant skills, experience or contacts?
If not: ditch it. If it is: embrace and continue the ascent towards your desired destination.