Fenix 6S pro vs Fenix 6 pro GPS accuracy comparsion:

Ok, now go out and do it a few more times, with a few more 6 Pro's and 6S Pro's, document all of the conditions and variables each time (how long did you soak the GPS before starting, did you wear both watches at same time and, if so, which one was higher on the wrist and did you try switching them around, etc etc.) and then maybe we can form an opinion about the results.

Until then, this information is not very useful for anyone.

Also, why didn't you use the Track Run mode that both watches have?

For a month now I have tried all conditions and all methods, with all hands, in all conditions, with all recording modes, all types of GPS. Compared 4 devices, 735xt, samsung phone, 2 fenix 6 pro devices. This post is not a complaint, this post is more of a quality conclusion.

K, but you only posted 2 results, and you only used 1 sample of each watch type so you can't rule out a hardware defect

Pretty meaningless without the tracks available for analysis, but it does look pretty clearly as though you have the 6 Pro Data Recording set to Smart while the 6S Pro is set to Every Second.

Can you export each of your tracks to Google Earth and show those results?

Quite possible. There were many posts about the accuracy of GPS of the same devices, when one person wrote that it does not work well for him, and another answered him that it works fine on this device. From the outside, it looked like a fault with the quality due to not knowing the features of the GPS signal. In fact, on the same devices, there may be a different quality of signal reception, and it does not matter if it is different hardware, or a breakdown.

If you are going to run on a track, the Track Run functionality will improve accuracy. 

www8.garmin.com/.../GUID-6F710D25-D03C-4C65-A42D-79475153F77E.html

Perhaps the size of the watch matters considering that the metal bezel is used as GPS antenna. Antenna would have a better signal gain when its size is closer to a specific fraction of the signal wavelength, for example 1/4. The GPS L1 wavelength is 19.05 cm.