</font><font color="blue" class="small">( I think the tool for *you* digging holes for 5gal trees would be the backhoe ... (when using a CUT). It might take a little longer (setting up for each hole), but a PHD in tough soil would be tough going (especially at 24") with a CUT. )</font>
I agree. Before moving across country, I spent plenty of time digging in rocky Connecticut ground with post hole diggers, and it was horrible. By the time you were done, you didn't have a nice cylindrical shaped hole. Broken equipment, shafts, augers, etc. were common. For soil like that, I think a backhoe would be a much better option, and you will get plenty of use out of one in your landscaping business. And like others have said, you will need a pretty powerful (and large) machine to run an 18 or 24" auger in rocky soil.
I also agree that the SubCompact sized machines would become too small for you pretty quickly. It has for me, and I'm only a part-time landscaper - if I was doing it for a living, I'd definitely be looking for a small Compact. That way you can have the smallest possible size with the biggest possible capacities, and not end up taking so much time to get the project done (as it will with a Subcompact like the Simplicity) that you end up losing money on every job. JMHO.