Shimano Deore XT is a brand...kind of like Toyota Supra.
Shimano offers both V-brakes and disc rotors in Deore XT.
I don't know much about Avid SD-Mag...that sounds like an old brake system to me.
There are a couple of advantages to riding disc....the main one being that they don't get all mucked up when you're riding in muddy conditions. They're also a lot easier to control and are more responsive.