Here is a somewhat recent article from a guy who reviews these things incredibly in depth. He tests 6 Garmins and a Coros here simultaneously and compares their accuracy. DC rainmaker gps comparison test
It seems the cheaper Garmins aren't quite as good in the more difficult city area but the stuff out in the open stuff looks pretty comparable to the more expensive ones. Much of it seems to be multiband vs. not.
If you're into a specific watch he probably has a review on it. Very in depth and I believe gps accuracy is something usually covered.