Unless you like the the grip texture of the Gen3 over the Gen4 or money is tight, go with the Gen4 MOS, gives you the most flexibility. If for some reason you ever decide you want to try an optic and have the Gen3, it will cost you a lot more to get the slide milled than the price difference between the Gen4 and the Gen4 MOS. Grip size, the Gen4 with the medium backstrap is the same as the Gen3.
Plus, you mention trying trying to fit a Gen4 mag release to a Gen3? Again, unless money is tight and you are getting a screaming deal on a Gen3 AND you have some skills to modify the frame, don't both jacking around with the Gen3. If you screw up, it will cost you way more the the delta between the Gen3 and Gen4 for a new frame. Plus, per the rules DFIVE3 posted, when a sharp RO catches the mods, if you get them to work, they will bump you to Limited 10 or Limited.
I have had many Gen3's and Gen4's, personally I prefer the smaller grip and mag release of the Gen4's. Oh, and if you do end up shooting USPSA, you will probably add grip tape to whichever you get.