I own a PowerTap Pro and I'm very very happy with it. As for which I'd buy? Well money not being an object I'd go with the SRM but you can kit out a good set of training wheels with a PTP hub and then get extra bike kits for your other bikes for less than half of what an SRM costs. I've got both my TT and RR bike set up and I just throw on my PT wheel and I can train on either. While the SRM is only going to be good on one bike. The SRM does allow you to use different wheel sets so you'll have your race info also.

Given the price difference the nod goes to PowerTap but it's close. If I had the money and I only rode one bike I'd say SRM but I could kit out three bikes with the PT for the cost of a SRM.