DIN valves are common in Europe, and are used for higher service pressures. A newer version of the K valve is the ball valve made by Scubapro. This valve allows more air to pass through and eliminates mistakes on valve opening and closing, but is more expensive. The older J valves have a built-in reserve, but with the use of submersible pressure gauges are used less frequently. Color - There's more to picking a color than fashion.
Some colors are more attractive for videography or photography.
Dive leaders often wear unique colors for recognition. Bright colors also make divers more visible to other divers, boats and people on the surface. Cost - In general, steel costs more, as do less common sizes. This tactic, selling a cylinder for a bargain price to spur the sale of more profitable gear, causes prices to vary widely between dive stores. Nothing beats a full tank of air when you're ready to stride off a dive boat.