Every area is different and you have to have a machine that fits your area, in my area, nobody has fences in their backyards, and not many care so much about the lawn as long as it doesn't cost a fortune to grind the stumps. It also depends on what kind of tree's are in your area and what size of stumps your talking about. If you have stumps that are hardwood and up to or more than five foot in diameter, a smaller tracked machine takes too long to get it done, if you have to get through gates and fences like has been mentioned and your stumps are soft wood and only a foot or so in diameter, either a pull type behind a pickup or else a tractor mount isn't really going to work the best.
The only one who can really answer your question is you, go with the size of stumps you have to deal with and also what the homeowners in your area want and expect. Everyone can tell you what to do based on their area, but that won't mean much if your area is completely different. For my area, large hp and big capacity and reasonably priced gets the work. Those running pull behind units with a dedicated engine either use their pickup to move from stump to stump or else hook it to a skid steer or small tractor to move it around the houses, or else they have a dedicated self propelled unit, tracked but the one with the bulk of the work always did it with a older JD 4020 and pto drive unit. It would not surprise me if he's passed away by now, I haven't seen him around since covid19. He used to grind stumps in the thousands if not tens of thousands of stumps per year, but the last I knew he was in his 70's and it gave him something to do in retirement.