Gopros are getting better at low light, but they are two very different tools. To film yourself, get a gorpo hands down. With a few people, dslr can give better footage if you put in the work.
Eg. Gopro
Put it on your helmet/chest/pole, you are good to go, might take a little time to learn optimal settings (which iso gives low noise, but bright enough to see). If you are filming your friends, it will be mostly follow cams, maybe a few shots where you can position yourself right (deck of jump for example), but expect to stay about 3 feet away when filming other people.
Eg. Nikon
You need a tripod or glidecam, good luck filming yourself (Ian Provo does it with a 5D, but that doesn't make it a good idea/mean that it is easy). You'll also need a fairly decent lens, at least f2.8 for night skiing, f1.8 would be better. The sigma 18-35 would be great, but it retails at $800, which may or may not be worth it to you. It will give you more freedom in your shots too, if you purchase additional lenses.
Essnetially, the gopro will give you decent results out of the box. The nikon will give you better results if you put the time and money in, and it will give you more room to grow as you progress. You don't need all the stuff at the start, a prime lens, a few memory cards, and some sort of stabilization will be enough to start. Because they are so different, many filmers utilize both to take advantage of both of their relatives strengths.
