I haven't seen 210 in real life, but IINM the 410 is much thicker than 410. But the 410 is not THAT big, like Timex Global Trainer. U can use it just like a regular watch.

The issue with 410 is the swipe movement. Sometimes U need 2-3 swipes be4 it responds. But during a run, if U wanna switch between views, U just need to touch the bezel. The watch will sometimes go haywire if water gets onto the screen & bezel. & never, ever swim with it. It's not designed to be used underwater.

The 210 is targeted either at runners who only wanted a basic GPS watch or the ladies (with its pink color etc). It's simple to use though.. at the expense you won't be able to customize the data fields (that's the fields that's displayed on the watch such as Distance, Pace, Time). In terms of size, the 210 is quite small and most won't recognize it as a GPS watch.

The 410 is larger than the 210 but slightly smaller than the old 405 (which it supercedes). If you are looking for the 405 features then the 410 is what you should get now.

However the 610 is really what you should be looking at. It offers everything the 410 has and slightly more (but I've to admit the "more" features of 610 is not really that useful so it's more marketing talk than anything else). The main reason you'll want to get the 610 and not the 41x series would be the touch screen on the 610 which is a whole lot easier to use - especially in the middle of the race.

The only downside of the 610 is the battery contact point corrosion that some runners seem to be encountering but that's a mixed bag as some seems to be fine with theirs.

The bezel in the 41x series was an innovative feature when it was first released many years back. It's still easy to use etc but do note the "locking" issue that Tenno mentioned ie when the bezel surface has sweat, it tends to confuse the bezel contact and become unresponsive to your touch. The 610 with its touch point method does not suffer from this problem (and one of the main reason I bought it, the 405 is now my backup).

One trick I used with my 405 is to bezel-lock the watch once I'm set it ready to start the race. In bezel-lock mode, you can still use the start/stop timer button... but somehow sweat, etc does not cause bezel lockdown.