I'd say the second one. There is more surface area for the sound to bounce off of. The purpose of a diffuser is to reflect and break up the sound. The first one is going to trap a lot of sound, and do who knows what with it. The second one has surface area for the sound to bounce off of.
I build some similar to the second one, and can attest to the PITA work it takes to make something like that.
Would be interesting to see a before and after report of a 2'x4' pair of the first one behind a set of speakers...might work better than you think it would
