I usually run with a frined that uses an Ambit 3 Run and I use the S9B (GPS only), in my opinion and comparing both watches I found the following:
In trail runs (using custom trail running sport mode), with around 300 m ascent very 10 km and some trees the Ambit 3 has most of the times a perfect track, the S9B has a very good track but not as good as the A3 and falls short on distance comparing the A3. Examples: S9B: 21.12 km - A3: 21.35 / S9B: 13 km - A3: 13.10 / S9B: 11.02 km - A3: 11.12 km
In flat runs (using custom running mode), here the A3 has teh same or falls short on distance comparing with the S9B, tracks are quite similar. Examples: S9B: 11.43 km - A3: 11.31 km / S9B: 11.32 km - A3: 11.30 km
I do not know if there is any diference in the sports mode that can affect this, but what I explain I have noticed several times.