Ugh oh, this is causing me to go away from yoke (after I just decided on it).. Ugh. It's game of: what is the likelihood of the extrusion occurring under water if I went yoke, in combination with, what is the likelihood that I will have difficulty getting DIN tanks for my charters and Caribbean dives if I went DIN.
Stop. You are WAY overthinking this. You are looking at unlikely events and drawing conclusions. The likelihood of a catastrophic yoke O-ring extrusion occuring u/w is VERY small. Even if it did happen, how long does it take a tank to completely empty with the valve fully open? Longer than it would take you to safely ascend from any recreational depth, even if you were near the end of your dive at 100 feet. If you want to worry about an O-ring extrusion, why not go ahead and worry about a tank neck O-ring extrusion - I have seen that happen (but I don't worry about it before / during a dive). I have also damaged a yoke-to-DIN adaptor through sheer stupidity - leaving the tank standing in a tenuous position and having it fall, and bending the yoke clamp on the adaptor. So, if you worry about damaging a yoke first stage, and decide on DIN instead, you still have the possibility of an issue if you use an adaptor on those boats that only have yoke tank valves. In OW recreational diving it is impossible to say that DIN is statistically 'safer', simply because the likelihood of a problem with a yoke reg/valve is so low as to preclude any valid numerical comparison.
How about this - the O-ring on a DIN regulator is on the first stage. It comes out occassionally, usually when you are assembling your gear on the boat (after the boat has left the dock and is motoring to the dive site), and gets lost. So, you don't see it fall out, you assemble your gear, and then turn your valve on and listen to all the hissing air. Does the dive boat have spare DIN O-rings? 'Uh, well, no, we don't use them, since our tanks are yoke.' Does any other diver have a spare DIN O-ring? 'Uh, well, no, I / we have yoke regs.' Did you remember to bring your Save-A-Dive kit, with a spare DIN O-ring, on board? 'Uh, well, no, I left it in the car at the dock.' So, you sit on board at the dive site, waiting for everyone else to enjoy their dive, which you can't do because that @#$ O-ring fell out. Unlikely? Yes, very. But, I have seen it happen. It almost happened to the mate on a boat I was on last week, except several of us had spare DIN O-rings (the mate didn't).
Personally, I prefer DIN regs and DIN tanks. All my tanks are DIN, most of my regs are DIN. But, I will dive both / either, as needed, and don't worry about diving a yoke reg to 180 feet (to pick a number). I do NOT like DIN-to-yoke adaptors, and will convert my DIN regs to yoke when I take them to the Caribbean (if you know what you are doing, and have the parts, it takes about 60 seconds). The point of saying this - in your situation, go with yoke, don't worry about it somehow being less 'safe', because it isn't. Spend your time learning good buoyancy and trim control, learning good finning techniques, enjoying the wonderful underwater world, and accumulating ideas for that
second set of gear a couple of years down the road.
Oh, and by the way - I wouldn't trust ANYTHING the bozo at the shop said, given the statement that 'the DIN reg will flow easier (since it has a larger hole) than the yoke.' With that level of ignorance, if he said that grass is green and the sky is blue, I would still want to confirm it for myself (while doing business with another shop). Seriously, I would not do business with that shop, even if everyone else there appeared to be somewhat knowledgeable.
