Not sure about sound quality differences, but I don't think they'll be too different. If they are different, the over-ear would probably sound a little bit better, considering they will cost more. The over-ears are also, well, over-ear, so I would imagine they're more comfortable. But I've heard many good things about the comfort of the on-ears. Also, the over-ears are going to be a bluetooth wireless headset. If I recall correctly, that means they will need to be charged, and I have no idea how long the headset will last on one charge. However, the over-ears should have an option to go wired.
I honestly wouldn't buy the Purity pros, the price is probably going to be very high and you can buy the on-ears for a very nice price of $99.