I'm glad I got a dual-sided power meter (Stages LR), because if I had gotten left-side only (and then it just doubles it) my power measurements would have been too high. Apparently I favor my left side by 3-10% depending on the difficulty of the effort.
How much are you willing to spend? The Stages LR is currently a bit over $700. A used single-sided power meter could probably be had for just a couple hundred.