About gps

GPS insta pace accuracy is a tautology. Seriously, GPS is accurate to only within about 5 meters, so one data point can be 5m behind you, the next 5m out to the side, and the next 5 m in front. While the algorithms can work out your most likely path and distance (and therefore average pace), real time pace from GPS is very difficult.

The best available option for insta pace is to use a STRYD.

All the devices you mention use the same GPS chipset, so there isn't much difference. Local conditions (eg. tree cover, buildings, narrow valleys, etc) are going to have a much bigger effect on accuracy than the specific model.

None of the modern GPS watches get pace from GPS only. Pretty much all watches fuse the data from GPS, accelerometer, and gyroscope to provide a more stable and accurate pace. That is called sensor fusing. But not all companies are equally good at algorithms. I'd say my experience with Suunto FusedSpeed was pretty positive. Coros also has something they call intelligent stride model. They claim they use machine learning to improve pace accuracy individually for each user. I can't say the same about Garmin's instant pace. Garmin still appears to fuse the data, but based on my experience with F6X does that quite poorly. 

For accurate instant pace, it's best not to rely on GPS. A well calibrated foot pod or Stryd will help you much better in that department. In case you are training and racing on a track, the Track run app does a remarkable job for instant pace once the track is calibrated. Having said that, when you train and race on a track, it's probably just as easy to simply check the stopwatch every 100 m (assuming that you know how many seconds per 100m your target pace is.