I've bought used Powertaps and SRMs and have always had good results. For $300 I would get a Powertap or two-sided crank based meter rather than a Stages. Keep an eye on the classifieds here as well as ebay and Pinkbike.

Powertap hubs are pretty bullet proof. They work or they don't.

I would search Craig's list as well and try to find one local. This way you can test it out before you buy.

I just bought a used Powertap Pro +, on a Mavic CXP rear wheel (including cassette and gatorskin tire) for $150 (including shipping). For what I'm using it for (trainer workouts), it works great. At some point, I can send it in and get it upgraded to a newer GS for something like $350.

Pretty good experience, IMO.

I've spent a bunch of time over the past month chasing after power (meters). Priorities:
1) Use it on tri bike on trainer.
2) Use it on road bike for rides with the local Roadies.
3) Use it on tri bike in races.

I chased after a lot of ways to skin this cat. Any solution needed to do ANT+. My stationary trainer already does power, but that didn't help me any with 2) and 3).

The cheapest way to get power is a used Powertap wheel on ebay. I got one for $200. So that's a solution for 1) and 2) that allows me to sell the expensive trainer with power and I can go back to my no-frills trainer.

I still hadn't solved 3) tho. It looked to me like a power crank solution was the way to go, unless I wanted to swap my old disc out for a disc with a Powertap hub. I found the latter on ebay for $1k which is a pretty good deal. But my old disc is so beat up that I wouldn't get hardly anything for it so the ultimate cost of the swap would be high. The crank solution wasn't infantry simple either. My bike used a BB30 bottom bracket which excluded a lot of possible crank power solutions. Also, I further limited the # of cranks that would work because I wanted to put Rotor QXL rings on the tri-bike.

Ultimately I found used SRAM power crank solution being sold by Amazon. Some kind of NIB that was returned to them, and is a BB30 and compatible with QXL rings.

Lessons learned.
-Used Powertaps deserve a look because they are available cheap. Some are old models tho, so know your Powertap model #'s.
-A Shimano BB and/or crank makes life easier because so many crank power solutions are compatible.
-WatTeam has an interesting DIY solution for $500. Limited crank compatibility now, but carbon cranks are supposed to be added to the compatibility list soon.
-Call up power crank mfrs and ask them about cosmetic blems that might be available.

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I have 3 Powertap hubs and love them. The GS, at $400 new, is less than I paid for my first G3 through the employee purchase program. I've had a few Stages. They were fine, but struggled with longevity. All were replaced under warranty.

If you're willing to live with left-side only, I recently bought the Praxis cranks with 4iii. So far, so good. Weight is very similar Ultegra 6800.

When I return to racing (next year, hopefully), I intend to put a Power2Max on the TT bike.

I agree on the PT Hub I bought a used G3 hub on craigslist in 2014 and it has been perfect ever since I think I spent 450 or so then and they were much more expensive than now.

Another vote for Powertap hub. I bought a G3 on a set of Mavic Open Pros for about £400 then one on a set of Enve tubs for £800.

Only attention they have ever needed is a new battery and that isn't often at all.