What kind of bike do you have ?
What level of components does it have ?
What kind of riding are you planning on doing ?
I have a powertap hub. Its pretty good, especially since I sent it back and it got fixed.....
Considering getting a Power2max for my race bike.
The complexity is the whole range of - where to put it, what exactly it measures, how much is it ?
On top of this there is component compatability for some of them - They are primarily aimed at committed road cyclists and therefore generally come fitted to decent level road components. Many of them come fitted to / for specific models from the main brands' road components. eg Stages on a Shimano Ultegra Crank, Power2max for a Cannondale SI crankset.
The most versatile - i.e fit to any bike - are the pedal based ones and the Powertap hub. Pedal based are road shoe compatible only.
Best reference I can give you is this guys site / general review - this tells you all really. http://www.dcrainmaker.com/2014/09/buyers-guide201...
He's not going overboard on the Stages.
Have a read and see what you make of it all.
and then ask more questions....