I've seen and used the 3" o-ring puck, and I think it's too big. Not so much from a diameter perspective, but it's just too damn thick (tall). It gets in the way, is too heavy, and frankly, isn't very attractive. And it's expensive, relatively-speaking.
The 'traditional' 2" dealer buttons are just fine, although I no longer use them. They are available in plastic (2") or ceramic (50mm).
I'm totally sold on the oversize 60mm and 63mm ceramic dealer buttons that have become popular over the past few years. 63mm buttons available from GOCC, and 60mm buttons available from OWPS and Sun-Fly. Best of both worlds, imo. Big enough to be seen (and allow larger artwork), inexpensive enough to be a non-issue, and not to heavy or bulky. I don't use anything else.