I've only helped with a 3 point auger. It was a bigone on a 75HP tractor.
After nearly killing ourselves while hooking it up after we unchained it from the fence, it dug a big hole. But it was crooked and we had to dig alot by hand.
Messicks has some videos of front loader augers that have actual down pressure compared to a 3 point.
In my limited experience I wouldn't buy an auger. Rent, borrow, or rent a skid steer.. 20 acres with 4 wire- assuming it's a square- doesn't sound to bad if it's barbed with T posts most of the way. My neighbor is doing 6 miles of fence, and has a gas t post driver thing.