Vortex Pro rings are vertically split rings, if you mean the newer horizontally split Pro Series rings I'd say if you are on a budget or just casually shooting at the range with only sporadic matches thrown in they will probably serve you just as well as the PMR's. If you want some of the best rings on the market and the peace of mind that they're machined true and are unlikely to fail then it might be worth the extra price bump. There are also height differences between the two lines in rings of the same diameter so that could factor into the decision too. 30 mm Pro Series come in .90, 1.0, 1.26, and 1.54 inch heights where the 30mm PMR's are .87, .97, 1.26, and 1.45 inches. Not a huge difference but if someone is looking to get their scope to a specific height it could come in to play. Also, if you're looking to save a little money on PMR's, search for the Seekins Precision equivalent, Seekins manufactures the PMR rings for Vortex, you can usually get them a little cheaper that way.
Good luck