I got to use both the designs that Luis had in Cayman. However, I have already told him what I think and he asked me not to say anything about one of the possible features and I am reluctant to say anything more beyond what I say below in an open forum.
My general comment about any DSV using a single hose mouthpiece is that they tend to place the hoses further in front of the face/mouth thus requiring the weight rings on the hoses to prevent the mouthpiece from levering upward on the mouth/palate/teeth. With the buoyancy of the hoses offset the Luis design was comfortable, the Draeger type, well, I was not especially impressed but it also continually leaked, sometimes enough I had to swim sideways to prevent aspirating water. Luis thought there may have been a damaged O-ring. Of course it did not leak with Luis (and then it did later

).
What if only the positive side had a valve?
Of course on the one dive in Cayman where I could have really used a DSV or the PMDSV, I had neither, go figure

. Surface swimming, loitering and shore diving, a DSV would be killer sweet.
Luis seeking Divine inspiration:
James