I would go with the PowerTap, absolutely no doubt about it. The cost of both systems is about the same, but if you want accurate power measurements then get the PowerTap. The Polar is just too way inaccurate. For instance, if you can TT at 280W the Polar can vary all over the shop, saying 255, 290, 320W so you're never quite sure what power you are training at and might decide to push harder or ease off. Only the PowerTap and SRM are truly accurate. I know a few Polar users that have invested (in fact one friend changed from a PowerTap) and all have been disappointed with the accuracy of the power readings (the other features of the unit like Altitude and the software are nice though) and two have now sold their Polar units. SRM is another option, though the PowerTap is more accurate (1.5%).