Last year I upgraded from a pretty old Vivioactive 3 to a 955 with a multiband GPS. The difference in GPS accuracy was definately noticable but didn’t really end up making a huge difference overall. The main practical difference is that if I am running in the open and at a constant pace for at least the last 15 seconds, I can see my current pace good to within 5 seconds per km (most of the time). With my old watch the current pace reading bounced around too much to really be usable, but the average pace over the last km or half mile was usually fine. My old watch would occasionally show a track way off when I was in trees or tall buildings and the new one doesn’t seem to do that, but that never happened enough with my old watch to bother me.

If you just want to see how far you went and your average pace, I think any of the newish Garmin or equivalent watches are just fine. If you really decide want the best GPS look at the ones with multiband GPS receivers (that is ones that pick up several different GPS frequecies as opposed to one GPS frequency plus Gallielo and/or GLONASS).