I own a Rayco RG13-II. I have had it for four years now, I purchased it new from the factory. Yes it is small but I can get into yards that the bigger ones can't, that's why I bought it. You make money by doing something that someone else doesn't do. Everyone around here has the tow behind.
It is a lot of hard labor to run the smaller ones but if you keep the teeth sharp and your back and shoulders are in good shape you will do fine.
The other reason for buying a smaller one is that I am a small company and I didn't want to go into major debt with a bigger one. I ground enough stumps the first summer to pay for it. I own a tree service and found that the stump grinder was the next step in the direction that I wanted to go.
The model Rayco that I have has held up like a champ. This summer was the first time that I had to replace the bearings on the grinding wheel axle.