You unlock both of them at the came time but it is VERY hard to unlock them!! Anyway:
~after completing story mode, you will see green mission signs instead of the usual red
~these missions are in all of the cities and if you want to see where the heck these are go to them menu and see how many missions you have completed out of how many there are total
~after you complete most of these missions they will join up with you
~if you want to complete the story mode you have to defeat eggman and bowser (again) but with metal sonic and bowser jr.
I HIGHLY suggest that you use metal sonic for snowboard cross because of his speed and jumping, and if you use metal sonic, it won't be all that hard.