I have limited experience with both. I used to think that clamping was better, but then I have changed my opinion. My thinking was based on the idea that clamping was applying pressure forcing blood in a smaller volume, where pumping sucks blood in the penis, but this way filling the penis mostly with fluids. So it should came as a consequence that pumping results should be mostly temporary while clamping gains more stable. First anecdotal evidence was supporting this idea.
But then guys started reporting that clamping gains are temporary as well; plus what I’ve read in medical reports about people using constriction device showed that pumping and clamping works about the same, both filling the penis with edema (not always easy to detect as such) that can take months to disappear.
So in the hand, I think pumping and clamping are basically the same, with pumping having some advantages mostly due to the fact that you can more easily vary the pressure applied and can be done even with a flaccide penis (although this is not ideal, probably).