I would suggest you find my article in Slingshot... The quick answer is, "no, it is not better".
Now, Blades pursuing is much better... the same for Pikes...They just should not be able to maintain contact with Ax. The article goes into great detail.
To answer the original issue.
I competed in several major US Tournaments with infantry mounted on 15mm that should have been on 20mm. Only one person noticed. These were large tournaments.
In the end. It really doesn't matter much.
This is true for the Bd differences.
Where you will see it most I think is with HYW armies. Here you will see dismounted knights rebased to 20mm to allow proper recoil distances. Again, this is a small change in combat value, but one that has some meaning.
In the end though, I don't think it matters. There has not been much complaint from the tournament scene. On the club or personal level... it doesn't matter.
I decided to only based 4Pk on 15 mm due to the fact than we often play them with the back support so if turn to flank they have one more recoil moove and then can stand better against blades. As we often play Asians Army or medieval westerners , which thus confrontation may often occurs.
I just discorver this thread, and i'm a bit surprised. As per the rules, i thought 15mm was the "legal" (ie for tournament) base, and 20 mm was just to fit if using some bigger figurines, but was not an "official" way to do.