For specific purposes you just have to try a model, a lot has to do with getting to know it. I know there are expert quilters who have set up both model 66 and 201 especially for FMQ, and do nothing else on that particular machine. I've seen them used as portables on a table as well as on those large frames. I don't think it has to be 15 even if they are a favorite. The models aren't all the same, hook, race, bobbin case set up are different even between horizontally as well as vertically set bobbin models.
Either way, you are onto it and the proof is in the pudding.