There are two styles of fitting for scuba regulators, international a-clamp and DIN. The DIN fitting itself then comes in two "sizes" of 200 and 300, with each being able to take 232bars or 300bars of pressure respectively but 300 Bar DIN is by far the most common. For the geeks amongst us... The 200 fitting has 5 threads and the 300 7 to cope with the extra pressure. Technically a 300 DIN fitting will work in both 232 and 300 bar DIN valves so most DIN regulators are 300BAR DIN by default.
Back in the 90's DIN was a New European fitting that no one particularly used. However today it outsells the traditional a-clamp 3:1. There is no denying the DIN fitting is more secure, the o-ring is trapped and highly unlikely to extrude as it can from the a-clamp fitting. Essentially an o-ring extrusion is when the o-ring between the cylinder valve face and the a-clamp fitting bursts and air escapes (usually underwater) rapidly, putting you into an out of air emergency fast.
So the question remains, why do a-clamp regulators still sell? DIN is more secure, can cope with higher pressures, is more reliable i.e. safer. Is it just familiarity? When a customer visits our store we tend to recommend DIN with a travel adapter, that's a screw on a-clamp fitting which converts the DIN fitting to the a-clamp. The only downside of the travel adapter is it makes the first stage protrude further forward than it normally would but only by an inch or so. It does offer the diver the security of knowing wherever he or she dives his regulators will fit the dive operations cylinders.
SHOULD I BUY A DIN OR A-CLAMP REGULATOR?
The obvious answer is DIN as with an adapter it can fit both types of cylinder. DIN is lighter, usually 100-200 grams lighter and physically smaller than the equivalent A-Clamp so is easier to transport overseas.