Let me poo poo the DIN vs yoke Oring myth a little. While I am sure people see more issues with Oring on yoke regs the problem is the oring itself or more correctly the treatment of the oring, NOT the type of connection. What is causing the oring problems in the yoke is the maintenance and care of the oring, not the difference in connection types. The DIN oring resides on the reg itself so they tend to get a lot less dives between replacement (annual service at least) and because the oring sees much less use on average. Most rental tanks do not have valve covers so the oring is exposed to a lot more environmental damage than a DIN one which is usually protected by some form of cap when not in use, not to mention yoke tanks get a lot less care from the renters. There is a widely held but totally inaccurate belief that DIN orings are "captured" in use and yoke are not. In fact, both types use the exact same seal design with one minor exception, where the oring retaining grove is....it's in the tank on the yoke and on the reg in DIN....most DIN proponents believe this but have not taken the time to actually study the designs.....I have. The only real mechanical difference between the 2 is how the seal is held together. Each does have it's advantages, yoke is more robust...it will take a lot more abuse than DIN, which is one of the big reasons it is widely used in rental equipment and as a result better suited to the traveling diver, at least in the US. DINs are more compact and are less prone to being knocked off or getting hung up but they are much more prone to damage, both the reg and the tank valve itself. DINs will handle more pressure but a moot point unless you are using your own HP tanks or renting from a tech shop you will not likely be using HP tanks.
The selection is no where near as black and white as many would believe. The solution to the problem is to simply carry a few orings, inspect the tank orings and replace as needed. Buying DIN because it's "better" then adding an adapter does nothing to solve the root problem - damaged orings....and in fact adds an additional oring/failure point to the mix. If you forget your adapter, you will be diving rental gear for the remainder of your vacation. As for buying DIN now for future tech use, its not really any saving. Most any regs to day can easily be converted from yoke to DIN at a later date and comparing the cost of the adapter to the conversion kit, the difference is minimal besides, you will most likely want dedicated regs for tech use if you go that direction. Considering all the gear and training cost you will have, the cost of another first stage is minimal.
Some of this is correct ... I'm going to take issue with some of it, however, regardless of how much effort you've put into studying the issue. I'll take it point by point.
First, the o-rings. Yes, you're right ... the difference is that the DIN o-ring is on the reg, while the yoke o-ring is on the tank. However, there's a significant difference in the thickness of those two o-rings ... and that effects their durability. I see yoke o-rings blowing out regularly. DIN o-rings don't tend to blow out ... although they do tend, on some regs, to fall out. Either way, it's an insignificant problem if you do what you were supposed to have been taught in your OW class and carry spare o-rings in your save-a-dive kit. That said, over the same number of dives (consider hundreds of dives being a reasonable sample size), you'd want to carry more of the yoke o-rings ... you'll be replacing them rather more frequently than you would the DIN ones.
Now let's talk about the design differences. While both designs fundamentally provide a sealing surface in the same way, there are some significant differences in real-life usage. And these get to the reasons why yoke regs are not acceptable for tech diving. In both cases the seal results from pressure pushing the o-ring against a flat metal surface. But the source of that pressure in a DIN reg are the threads on the inside of the tank valve ... which apply pressure evenly as the regulator is screwed into the tank valve. On a yoke reg, the source of that pressure is a screw on the back of the yoke, which applies pressure ... literally ... at a single point. It is therefore easy ... and common ... for the yoke to be mounted to the tank slightly unevenly. In most cases it won't matter. In some, however, it creates an unevenness of pressure sufficient to extrude the o-ring, which is the most common cause of o-ring failure in this setup. Furthermore, banging the yoke knob during the dive can cause this problem, which will result in ... at best ... a leak in the seal. At worst, the o-ring will extrude sufficient to cause a more rapid loss of gas ... or it will simply blow and cause a freeflow. I've seen all of these occur at some point or another with yoke setups. DIN is ... by design ... the more robust form of creating the seal, because the manner in which the o-ring is sealed makes it much less susceptible to uneven pressure over the sealing surface, and therefore dramatically reduces the problem. I have never seen an o-ring on a DIN regulator extrude due to the first stage being banged against something during the dive.
You said "yoke is more robust...it will take a lot more abuse than DIN" ... this is definitely NOT the case. It's far easier to damage a yoke reg, in part because the flat sealing surface is more exposed and therefore more susceptible to dings and scratches, and in part because of the design weaknesses I mentioned earlier. Again ... that is why every tech diving organization in the world mandates DIN regulators. It's also why ... as you mentioned ... DIN regs are designed for higher-pressure tanks.
There are ... as with all things ... other practical considerations. And you mentioned some of them. Rental tanks tend to be yoke, and that is primarily due to cost. So if you're primarily traveling to dive, or don't own your own tanks for local diving, a yoke reg will probably give you greater bang for your buck than a DIN. But we're seeing more and more convertible valves on rental fleets these days ... and those come with an insert that allows the tank to be used for either DIN or yoke. It will take some time for the older style tank valves to get replaced by these newer designs, but at some point reg style will no longer be a consideration for rental tanks.
I'm not a big fan of DIN to yoke converters. As mentioned, they tend to make the first stage stick out to where it can become a distraction by banging on the back of your head when you look up. I much prefer DIN regs, but do have some yoke first stages that I use when I travel. It's my ideal solution to the problem. Not everyone will want multiple first stages, and so factor that into your buying decisions ... but please, make those decisions based on factual information. How much you want to make out of the potential for o-ring failures is up to the individual ... but DIN regs are, due to their inherent design, more reliable than yoke regs when it comes to maintaining a sealing surface. If you want a demonstration of this, it's simple enough ... take your yoke reg and go to pretty much any popular dive destination. Put your reg on a dozen tanks and leak test them in a dunk tank. I guarantee you that at least half of them will have some measure of bubbles coming out of them. Those bubbles are due to o-rings that aren't sealing properly. This is often due to a lack of maintenance ... and can be fixed by replacing the o-ring. But do you really want to be replacing o-rings on rental tanks that someone else will be using on the next dive? At least with a DIN reg, when you replace the o-ring it's on your reg ... and the next person who will be using it will be you.
... Bob (Grateful Diver)
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