Pete and northwoods diver pretty much got it but there are a few things to add.
For clarification, 200 (actually 232 bar) DIN valves can accomodate a K valve insert to allow the valve to be used with a yoke reg, a 300 bar DIN valve cannot as it is two additional threads longer and is too long for the yoke to fit over it.
The longer 300 Bar DIN valve is also however more damage prone as if the tank falls over the longer unsupported section is likely to be bent to more of an oval shape.
300 BAR valves are not stronger than 200 bar valves, both woudl handle the 4350 psi allowed by the 30 bar valve, the reason for the difference was to prevent lower service pressure 200 bar reguators from being attached to a 300 bar tank as the deeper 300 bar valve would not allow a 200 bar reg to seat. 5 threads or 7 makes no real difference as the odds are not more than 3 threads are in full contact anyway.
If properly designed and executed a yoke valve o-ring is also fully captured. The problem arises with lightly constructed yokes that may stretch at very high pressures and more commonly with yokes not being securely tightened. When that happens the extra room can allow the o-ring to extrude. However the same thing can happen with a DIN reg, and on an unpressurized deco reg a DIN valve is in my experience slightly more likely to become loose than a yoke reg. The difference is the location of the o-ring and what happens to it. On a yoke valve, the o-ring is on the face of the valve and will be blown off. With a DIN valve, the o-ring is seated in the regulator and usually does not go anywhere allowing the reg to be easily resealed.
As pointed out previously, the lack of a yoke knob greatly reduces the tendency to snag line and other potential entanglement hazards.
So in short, I am believer in the 232 bar DIN valve as it allows the use of DIN regs on 3442-3500 psi tanks and the convetible aspect of the valve allows it to be used with yoke regs as well. The 3442 versus 3500 psi limit is arbitrary but with the virtual dissapearance of newly made 3500 psi tanks, it will I think be a moot point in the future. Similarly, the increasing commoness of the 232 bar DIN/K valve will eventually solve the "traveling diver will encounter a yoke valve" problem.
Consequently, I don't forsee a situation where I would buy a valve that was not of the 232 bar DIN/K variety.
