Tough choice. Knowing what I know(which ain't much)... I'd go for the largest engine you can get in the frame size you are looking at if at all possible.
Kubota makes some fine tractors. So does New Holland, Deere, Cub, and a number of other manufacturers.
In a pulling contest, the conventional tractor would win.
In a re-sale contest, the conventional tractor would get a better price.
In the longevity contest, the diesel will last longer than the gas engine in the 400 series PT.
With that said, I think in a real world head-to-head chore contest contest between a PT and a conventional tractor in a typical home owner's situation, the PT would be more versatile and get jobs like mowing, snow removal and material handling done much faster than a conventional tractor of similar size. Mini hoe work cannot be done with a conventional tractor. Post hole digging with a PT VS. a conventional tractor is no contest, either. Box blading, I think, goes to the conventional tractor, because it is a pulling process, although I have no hand's on experience with a box blade and a PT.
Ask any PT owner that also owns a conventional tractor and they will tell you that the PT style is better for most things and the conventional style is better at a few other things. The hardest part is deciding what chores you will be doing and which style tractor is best for you. Don't jump into any purchases until you have those tasks clear in your mind and you have tested both styles of tractor doing those chores. The goal is to get the machine that best fits your needs.