There is really no comparing snowboarding boots with ski boots. While both are used for downhill winter sports, the only thing they have in common is that they help to strap your feet to a sled. Snowboarding boots are worn exclusively for downhill snowboarding. Ski boots, on the other hand, come in several distinct styles include downhill, cross country and telemarking boots. Despite the fact that downhill skiiers and snowboarders share the same slopes, different boots must be worn for each of the sports. In other words, one may not use downhill ski boots for snowboarding or vice versa.
Limiting the comparisons to downhills ski boots with snowboarding boots, there are a few similarities. Both types are worn for downhill use. Both attach securely to skis or a snowboard by means of a pair of bindings. Skiiers use two skis, one attached to each foot, while snowboarders bind their feet onto a single sled. Beyond these few similarities, the differences between the two types of boots begin to mount.
Both ski boots and snowboarding boots lock into bindings in order to keep the rider's legs firmly attached to the skis or snowboard. Ski boots lock in with the rider placing the toe of the boot underneath a flange on the front binding and snapping the heel into the back binding. The boots are not meant to detach from the bindings, although it does happen during a wipeout. Snowboards boots lock into bindings as well, but instead of snapping into place, the rider places their foot into the binding mold and fastens the board to their foot with adjustable, cinchable plastic straps. Snowboarders commonly remove one foot once at the bottom of a run in order to board the ski lift, for they don't have the luxury of poles like skiiers.
Material and Firmness
Ski boots are constructed out of hard plastic that, aside from the moving parts, is entirely unbudging. The inside is lined with soft, thermal material for padding and warmth, but they allow for hardly any ankle movement. This design is a practical measure to prevent injury while skiing. Snowboarding boots more closely resemble durable hiking or work boots in that they are more flexible. They have tread similar to walking boots while ski boots do not. Snowboarding boots are arguably more comfortable than ski boots, but, again, the nature of skiing demands that boots be extremely firm and allow no movement.
While the sports are very different, each requiring specific gear, one area that snowboarding boots are at an advantage is walking. Whether its to and from the car, going to the bathroom or the cafeteria, snowboarding boots allow riders the same freedom of movement while walking as normal boots. Ski boots, due to their rigid design, force skiers to walk in an awkward heel-to-toe motion, with the toe falling to the ground with each step. It's easier to unstrap ski boots when walking, for it loosens them up and allows far more ankle movement.
Depending on your downhill winter sport of choice, the type of boot you'll have to wear differs widely. Given the gear requirements for each sport, you'll have to settle for the particular boot regardless of the comparative advantage the other type may possess.