I have only incubated once but here is my brief story. After fairly intensive research I settled on a styrofoam farm innovations circulated air unit. I made this decision based on the unit itself. What I mean is I purchased the bator, brought it home and spent two weeks getting it to maintain temp. I learned what variables made the temp rise and what variables made the temp fall.
My first incubator (which I returned) was still air and the reason I returned it was I couldn't maintain a consistent temperature. There are lots of good incubators out there but in my opinion it comes down to testing the unit that you receive. Even the best incubators can have problems and not maintain temperature.
Whatever you decide, run it for a few weeks to make sure there are no issues and that you understand how to use it, this way you can react correctly if there's a problem and save the eggs.