Foam helmets are interesting to work with. I made a Mk.VI suit from Halo 4 completely out of foam. The helmet looks good but you need to be extra wary of slight warping of the overall product due to imperfect cutting of edges. After making pepped helmets and foam helmets, I don't think I'll make a foam helmet again.
Pepping gives you more detail.
I'm not saying it still won't look good, but you do lose some detail.