VMG much better. Polars are better for yachts, and due to the masses of lead they drag around can be sailed close to their max and it is harder to drop away from targets.

Cats on the other hand can accelerate/decelerate due to a multitude of factors. Training with the Velocitek it is normal to see a +/-1.5 knot difference in boat speed. Yacht would be +/-0.2 knot from polars.

This is prob not technical enough for many of those on this forum, but just my opinion from sailing various types of boats and using various types of devices.