I think the difference is where the pressure is coming from. With Clamping it comes from within. With pumping it comes from the outside.
When I’m clamped and fully hard I’m at 1/4” more girth. I have been clamping for a couple of weeks now and there seems to be a difference but it might just be in my head.
Anyway I’m going to try it for a month along with Jelqing to see if there is any girth gain.