I don't have experience with Diamondman, but DQ events are IMHO sort of haphazard. My first half was WTC and my second half is also WTC, the Eagleman in June. I've done two DQ events and both were unimpressive. The bike racks didn't have numbers on them, the guy giving the pre-race for both was either too quiet, disorganized, or didn't have the right info. I'm doing a DQ in Sept at Marlton Lakes simply b/c I like the venue, but their races seem to me to be sort of econo-mode.
I've done both. Finished Diamondman (first half), DNF Shoreman last year. Shoreman race area is more compact (transition close to water, easy to bike in/out, run in/out, close to parking and spectator friendly, but they ran out of water on the run course during a REALLY hot day. Bike courses were similiar, though Diamondman was an out and back (big "hill" aka a bridge) and Shoreman was a looped course (very flat). If I were do to it again, I'd probably choose Shoreman mainly because it's closer to my home and hope and pray they bring enough water this time.