Hey Alex,
I guess I’m the best person to answer this as I use DNA Pro hard on both sides as well 🙂 I don’t see any benefit in changing to 1.9, as long as you find you have enough control with the 2.1 sponge, the only reason to change would be if you felt the rubbers were too quick and springy and wanted more control with a thinner sponge.
So if your happy with the feel and control stick to the 2.1 sponge
Cheers, Tom
