RE Stages - before you buy, try and have a go on something that measures L/R power like a WattBike or something with Vectors. As I understand it (having read up on it, talked to people about it, etc) Stages is great if your L/R balance is close, always within a few percent regardless of effort level. It's less great for people where the L/R balance is off when bimbling along but evens up under load, or vice versa, it's fairly even until there's a lot of load being applied at which point one leg becomes dominant. Because you're getting power from one side, for these cases it's not a fair representation of the actual overall effort you're putting in - especially if your dominant side is your right leg. That's not to say you wouldn't get value from training with power from one side, but target power for intervals might be off because the leg being measured might be putting out different power to the other so you might not be getting the full intended training effect from a given session.
I'm not sure that's a great description... definitely worth talking to MtB about it though.
One other option - Garmin have just announced the Vector S - single sided Vector for slightly more than Stages which can be upgraded to dual sided by purchasing the extra pedal. Helps spread the cost if you do it that way, although if your L/R balance is not even, then it might render historical data that you collect from a single side as misleading.
Having said all that, if you can verify that your L/R balance is very close, then Stages or Vector S represents a brilliant low(er) cost approach to training with power.