Join TrainerRoad.com
It costs all of $9/month. You'll need a speed sensor, which you might already have, or only costs about $30 if not (when you do get a power meter, you should use that for TrainerRoad).
Doing a series of power-based workouts will give you immediate body feel feedback to correlate with the watts numbers you see.
Also, your power data is best interpreted in relationship to your FTP (functional threshold power). If you're doing TrainerRoad workout plans, you'll always know what your current FTP is.

Look at the buyers' guide and reviews on DCRainmaker.com
Ray has the most in-depth info and comparisons of all the power meters on the market.
He's a former technology consultant, and did fitness tech testing and reviews on the side. Now his info is so sought after the DCRainmaker.com website has become his full time job.
It is said, with no exaggeration, that manufacturers get his beta or pre-market review because they don't want to put a product on the market if Ray finds and reports a problem with it.

The major brands of strain-gauge power meters are all as good as you will likely ever need. There are slight individual differences that might make one or the other a better choice for you, but they're all plenty accurate for any practical purposes.

A couple things to think about:
Location: Power meters are classified by what location on the bike they pull their data from. Rear hub, chainrings, crank spider, crank arms, bottom bracket, pedals. There are some minor technical performance differences between the locations, mostly of interest only to engineers. Base your decision on practical factors; for example pedals are pretty easy to switch from one bike to another, so if you want power on multiple bikes that may be a good choice. If you like to change wheelsets on your main bike, you won't want a hub-based power meter.
Head Unit:
If you've already got a power-capable bike computer, make sure you get a PM that's compatible. Some PM's broadcast on Bluetooth as well as ANT+. Bluetooth is more likely accessible through your phone which can come in handy in some situations. ANT+ is a data broadcasting format that's mostly used by fitness tech devices, although there are a few phones that have built in ANT+ reception.

If your bike has a groupset that either Stages or 4iiii make a power meter crank arm for, this is probably your most cost-effective option.

Don't worry about left-right power balance. Most riders have slight differences right to left, but unless you're doing some kind of injury rehab you can't do much about that nor need to.

I personally would not fool with the power meters that are based on a wind speed measurement pod. To my mind this is the one product on the market that presents significant accuracy issues. Also, it doesn't work at all on a stationary trainer which is (arguably) the most useful application for a power meter.