Diesel is nice. As far as I know there are no Diesel ATV models - and the power take off is nice too- eliminating the need for a separate generator. However, the base price is about the same - 550 cc Polaris Sportman will run around $9000 list, so yeah, this is all about what ELSE you want to use it for.
The other thing to consider is that most ATVs are belt driven - so your ultimate pulling power with an ATV is limited by the belt slippage - so even with a lower HP motor, the tractor might be able to plow better in unbroken ground based on having a direct transmission. However, a 550cc ATV should be able to pull at least 1000lbs. (stopping it once you get going might not work out however) But given belt slippage, and new ground, multiple passes may be required with an ATV , or may require a real tractor to come in and break up the ground the first time.
Also found
these. They come in both tractor and ATV models..
I think either path will work for small scale farming. I personally will have a solar system with Gen backup for Electric generation, and I don't think I will have a need to run electric devices in remote locations ( and if I did I could haul the generator out there) with the ATV
A friend of ours did a lot of research on ATV farming and then decide he wanted to do more than a small plot, and bought himself a Kubota and real farm implements to use with it. We are sticking with ATVs because a lot of our teotwaki planning is done under the assumption that we may be wrong, and that everything will be fine. So the Teotwaki place IS our retirement home. We get ATVs and we can still use them for recreation purposes, even if small plot farming is never resorted to .. etc etc.