Ive been using my dainese ski helmet for biknig recently i dont know why i just prefer it, its so much more comfy.

Personally I would stick with the correct helmet for the correct sport, though I am sure that my SB helmet would suffice for riding I am unsure whether they are subjected to the same tests, and if not which is tested to 'lower' protection levels.

SB helmets tend to have less vents (for obvious reasons) so will be a bit warmer, the ear pads tend to be removable, plus SB helments tend to be more expensive (especially the p*ss pot style) as they have more 'features'.