Tow behind VS. Self propelled

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I currently own & operate a Vermeer SC352 stumper. It just is`nt cutting it when I tackle the big stumps out there. We are thinking about purchasing a tow-behind unit. Having never even used one before, I`m not sure what I`m looking for. Are the older 665/665 models much better than the self propelled 352?!? Do the newer SC502`s blow the others away? I dont want to spend more than $10,000, and I also dont want to buy a lemon that isnt much better than my 352. Any info will be appreciated.