Jay,
An inverting set simply means that the base is higher to accept punches to be used as stumps. A non inverting set has a lower base and can accept only the small button like stumps on the die plate. Inverto was a brand name offered by the now mortibund K&D company. Other makers of these tools offered the same technology but could not use the trade name. From what I have used and blogs I read, the best staking sets are the ones that were made by American Manufacturers. K&D Inverto sets are the most sought after sets and can be expensive. G F sets are excellent and can be purchased for a lot less but their stakes are made from 3/16ths drill rod (.188 dia) and the other manufacturers used #12 drill rod (.185 dia). The G F stakes and stumps are not interchangable with the other sets. If you really want to save a lot of money you can put a bid on ebay for a set with some rust on it. You can probably pick up a rusty set for around $50.00 or so. The rust can then be polished off in a lathe with Scotch Brite and motot oil. As long as you don’t sand the stakes and use only Scotch Brite the diameters will not be affected. I have done this many times.
David Pierce