I remember the first time I heard the brand name "Garmin". I wasn't even a runner yet, and it wasn't about their watches. It was at least five years ago, and I was thinking of getting a car GPS unit. A coworker had one, and recommended the brand.
Now, a reference to one's "Garmin" immediately means to me "running watch". Completely synonymous terms. I actually own two: I bought the Forerunner 210 shortly after my first half marathon, once I decided I really was enjoying this running thing! and got the Forerunner 10 this summer while marathon training, when I was worried that the watch battery wouldn't last the entire 5+ hours I expected to be running (the 210 actually ended up lasting for my entire 6.5 hour marathon).
There are other brands of GPS watches: Timex, TomTom, Polar, to name a few. I mostly picked Garmin because I did like the car GPS unit I got by them.
Both the 210 and 10 have two screens you can flip through to see data while running (in addition to the main clock screen). There is some degree of customization of available, but what I have showing:
Screen 1: total distance, total time, current (rolling average) pace
Screen 2: lap distance, lap time, lap pace
Screen 1: total time, total distance
Screen 1: total calories, current pace
Obviously, the 210 gives you a lot more (and, in my opinion, more relevant) data while you're moving. However, the 10 gives more relevant data in a condensed form when viewing history of a run afterward:
Screen 1: total distance, total time, total calories, average pace
Screen 2+: lap distance, time, calories, and pace, for only 1 lap at a time
Screen 1: total time, total distance, average pace, total calories
Screen 2+: lap pace for 4 laps at a time
Both watches are comparable in a lot of ways. They both track the same data (distance, time, pace, calories), so you can find the same data somewhere from either, it's just a matter of whether it's readily available during a run, or if you have to find it on the watch or after uploading to Garmin Connect afterward. Both can be set to auto-lap in any increment you want (I normally to 1 mile, so I can track mile splits automatically).
The main, and pretty much only, thing on prefer on the 10 is its aesthetics - it's smaller and a more fun color. I don't like that you can only see two pieces of data at a time, and that one of those apparently has to be calories. I sometimes refer back to how many calories I burned in a workout if I'm tracking calories for the day, but that would be later. Why in the world would I need to see calories while running? Are there people who just run until they've burned a certain calorie goal? That seems rather messed up.
The 210 isn't that much larger, and has more data per screen while moving. I know, thanks to the marathon, that the battery life will last at least 6.5 hours (the newest releases from Garmin have color, which make me concerned that their batteries wouldn't last nearly as long)! It's fairly mid-range (as far as Garmin's watches go). It has all the basic and semi-advanced functionality, even though it doesn't have all the bells and whistles that some of the more expensive ones have (like a "return to start" option to retrace your route back to home - that was one that made me seriously consider going a step up!). But it has everything you need that make buying a GPS watch worthwhile in the first place, and more convenient than using a phone app.
I've had the 210 along the way for a good chunk of my running experience, and I expect to keep it around for a lot more of it. Of course you don't need a GPS watch, but having it, and all the data that it collects, really made me feel like a "real" runner!