Your options would be
Powertap
SRM
Quarq


IMO, crank based is the way to go, I love my SRM. You can run whatever wheels you want, they're lighter and very reliable. You can get a used SRM off ebay for the price of a new powertap.

First buy this book to see if training by power is really something you want to do. Putting a powermeter on your bike won't make you faster without a lot of work.