Eh, they're all becoming interchangeable. There used to be more difference in the protection at the back of the head but a lot of models come in both and road and mtb with a removable visor being the differentiator.

Decide whether you want a visor and how many vents. I hear a lot of complaints about heat and weight with the more limited vents. I personally like a visor when I'm mountainbiking but don't when I ride on the road (at least in the hills).

I just bought a Bell Sweep R and really like it. The shape is ever so much better for my head (a bit rounder than the Giros I've been wearing) and I like the dial thingy for adjustments. My length of my hair can make a big difference in the fit and then add variable layers for the winter - it's a lot easier with the dial than the Roc-loc to loosen and snug each time.