It would be a pain to transfer the Polar Power between bikes. It involves removing a chain speed sensor, wheel speed sensor, power sensor, and wired bike mount. After getting these installed properly on one bike (involves measuring distances between parts of the drive train and weighing the chain), you'll not want to touch it.



You could get a HAC4, but the method of determining power is through estimates--not very accurate.

The PowerTap (PT) would not be good, because it's mounted on a specific wheel. You said you wanted it for on and off road--you'd need one on a road wheel and another on an MTB wheel. The cost would be prohibitive at about $1,000US a piece per wheel.

The ERGOMO is installed as part of the bottom bracket and the SRM requires a custome crank and chain rings... so, you'd need two of each for each bike. ERGOMO - $1,500US; SRM - $$$$