I started my buisness with a post knocker on a 1.5 ton digger, it was fantastic, i had and auger with it too and if i knew the ground was going to be hard i would pilot drill all the strainer with a 4" auger prior to knocking them in.
It was a great set up really as i could tow the whole lot on the trailer behind my land rover, and on smaller jobs i could take enough materials for 200 meters of fencing in the pickup whilst taking the machine too.
I could sneak it in through woodlands, reach over ditches walls etc and go up footpaths etc. 3 years later i decided to buy a tractor and postdriver- a pretty decent knocker with a rockspike and telescopic sideshift etc. I now have a 3 ton digger with a Protech on too. I miss my 1.5ton setup so much that i have now decided to sell the tractor and postdriver and get a new digger with all the fencing attachments. I can nip around and do all the small jobs with it and i will keep the 3 tonner for bigger/long term jobs (mainly because i have to transport the digger and postknocker seperately due to legal weight issues behind the truck.
Also another pro for diggers, you have all the buckets with you for pre grading/pulling out old fences and i always get asked to do over jobs with the digger "whilst its there" like ditching etc.
If i was doing 100% fencing day in day out pretty much, id get a tracked knocker, but they are heavy to tow around, and a definite one trick pony with no ability to load materials or prepare gound etc. I have also be told by people who have the 3 ton dumper converted one that they are not that stable if you have to fence sideways with the weight downhill, they don't have much of a counter balance as standard.
I did like the idea of this little machine for small jobs http://www.wrag.co.uk/wp-content/gallery/gallery/IMG_20150703_121930.jpg