I have an iPhone 4S and Cyclemeter. I'd like to start monitoring my heart rate and cadence.

There are Bluetooth heartrate straps and speed/cadence sensors (I'm looking at the Wahoo units) and there are also ANT+ straps and sensors plus an ANT+ key dongle for the iPhone. Can you recommend which protocol to choose? Are there brands you prefer?

I'd like to have the least drain on the iPhone battery, in addition to simple and reliable operation. Thanks!

Bluetooth is a power hog, and your phone will not last long, probably about an hour from a full charge, displaying HR info and communicating via BT.

I don't know what the ANT+ dongle uses powerwise, but I'd still think going ANT+ is better so that you've got connectivity in the very likely event you want to get a dedicated cyclometer (and leave the phone for calling), or add power and cadence info to your training.

The firm iBike make an iPhone case that mounts to the stem and has space for a backup battery, so that may be worth looking into. The also have an app that allows split screen nav and biometric readings.

FWIW: There is a new "Bluetooth 4" which uses much much less power and is suitable for cadence sensors, heart rate monitors, etc... Devices using it are labeled "Bluetooth Smart". It's starting to appear on devices and phones...

https://www.bluetooth.com/Pages/low-energy.aspx