I've been using the MapMyRide app for iPhone 4S, and it drains my battery pretty quickly and consistently over about 2-3 hours. (It does help to manually shut down the app after riding and saving your data- just "X" it out after double-clicking on the home button-- otherwise it will continue to drain the battery.)

Anyway, I was thinking about the Wahoo ANT+ sensor, but due to the battery life issue, I'm considering a Garmin HRM and external bike sensor. I'm also thinking that when I change phones in a year or two, I don't want to have to deal with new bike equipment (unless I want to), and the cost is about the same to have a separate Garmin+sensor solution.