The prong size is one thing. But there are a lot of differences other than that. If you are looking at tillage equipment the other differences matter. For example, the Farmall 300 typically has a Fast-Hitch with a hydraulic cylinder for draft (pitch) control. The Farmall 350 has a Traction-Control Fast-Hitch. Implements differ depending on those two hitches. For example, you move a disk harrow from a 300 to a 350 and you may find the front in too deep with the back not deep enough, or maybe the reverse. The 300 Utility is even more different, using a top link similar to a 3-point hitch. If you are just going to use things like a platform carrier, it doesn't matter.