What features should I consider when getting an ice cream or gelato machine?
You covered the big differences. Everything else is going to be brand and manufacturing quality.

My opinion on the different types:

For small amounts and small budgets I really like my cuisinart with the prefrozen bowl. The bowl gets so cold in my chest freezer that I don't even have to take the time to chill my mixture if I'm in a hurry. It always has plenty of cold leftover.

The ice/salt type either hand cranked (that's what kids are for) or electric are nice if you need to make batches back to back. You can obviously refill them instead of refreezing a bowl.

I don't have experience with the self refrigerating variety as they are out of my budget for the occasional homemade ice cream.