I think it is a matter of how large an auger you need to use, if I were considering 12" or smaller the PA15 is ideal. For large holes the PA30 can handle up to 36" bits. I build alot of heavy wrought iron gates and these require a large footing to secure them in place, that is why I decided on the larger unit.
As for whether the unit should be fel mounted or backhoe mounted depends on the equipment features. With and excavator or 110 you can drive from one position easily and reposition for each hole, this works well. If your backhoe necessitates that you climb down back and forth from the front seat to the rear seat that could be an issue. If you have a spotter then the fel mount can work well too. It was also mentioned that you could offset the drive unit to one side on a fel mount so you could see the hole, but the problem with this is you are twisting and stressing the loader frame, not good.
When I considered the options I had the backhoe mounting with the 110 seems to make the most sense to me. Using the backhoe mounting also provides more reach which I have found to be very important.