Just remember, the Powertap C1 needs 5 bolt pattern spiders, so 6800,6900 Ultegra or 5800,5900 105 is out. Finding a 6700 or 5700 165mm crank is next to impossible for a reasonable price.

Personally I like the C1, it works well. However crank compatibility is a pain in the rear.

If swapping between the two bike, I think your options with a Sram crank with either the Quarq or a C1 are your best bet. Bottom brackets are cheap relative to the whole purchase. You do need to be careful, there are bikes out there that may not work with a 30mm spindle as a 30mm bottom bracket may not fit.

Edit: One other note on Shimano cranks. Due to the way the left crank arm attaches, I personally don't think you want to be swapping it on a regular basis. Constantly tightening two bolts that hold the arm on could be come a problem with regular tightening.