I just went through this very decision and purchased two weeks ago. I tried both the 4105 and the 4120. Went with the 4120 - absolutely no comparison. Others have already discussed many of the differences. The bare 4120 weighs about 800 lbs more than the bare 4105 - says a lot about how it's put together. The 4 cyl Power Tech Diesel is a beast. I paid 16.5% under list - about $5K more than the 4105, but I have no regrets. Best tractor I've ever owned and it will do everything I will ever need to do. TONS of power - way more grunt than the 4105. Hope this helps.