Go for the Powertap Pro or Powertap SL. I just installed my Powertap SL. The SRM crank, even the Pro version, requires frequent calibrations. The amatuer one is even more inaccurate and need more frequent calibrations. The Powertap is easier to install but you are restricted to your wheel choice. They are as accurate as the SRM and Polar. Not messier than the Polar power setup. If you go for the Powertap, go for the SL just for an extra $200. Better wet protection and better bearing. The Powertap Pro is known for not working when wet and the bearing cone is too cheap. For the SL, more online vendors have them in backordered due to supply and demand.