well i see no one else has responded yet so i'll chime - i tried on several suomy's w/ duc custom/superbike paint jobs. loved 'em all but could never get a fit - my big-a*s head is too long front to back - got an arai - great fit. far as protection goes i believe them to be on par as good as it gets. fit (obviously) is the first step in protection so i went w/ the arai.
I tried on Arai in the shops around here and they felt pretty good. I heard people tell me the fit was similar between the two but I liked the Suomy paint schemes better and ordered one on-line. When it came in the fit I thought was a little better. It all comes down to what fits best for each person. I love my Suomy and will get nothing but personally.
When I was looking to replace my Arai Quantum, I tried on a couple of Ducati (Suomy) Supersport and Superbike helmets, and found they didn't fit very well--meant more for a round-headed person. But I've since bought a Suomy Spec 1R which is a much better fit, but of course not Ducati branded.
I have the Ducati Soumy "Scudetto" model. It fits perfectly, I was used to wearing Shoei helmets in the past. This one was brand new and came with my bike. After about 2 rides with it I thought it was the best helmet I had owned.
It is lighter than my Shoei and seems to be better constructed.
Arai helmets never did fit me quite right.
I have several Suomy helmets. The fit was good but quality is poor. The first one I high sided in at a track day. The second one that I am wearing in the bike feature pic I won at a drawing at Ducati Island at Laguna Seca. The side pieces fit very poorly, and if the shield is flipped up the pieces have to be popped back in. The orange stripes faded to almost nothing and they would not cover it under warranty. ( My buddy had an Arai replaced 2x, due to paint fade) The helmet was always kept in a storage cabinet out of any light. The one display at the local shop has been out for several years with no paint deterioration. My crashed one was a year older and its orange has no fade. I sent several photos and received a sorry letter from them and I sell their helmets. I always recommend Arai now, they have won my vote, and their customer service is available at most races and through local reps. The only reason I have even kept it is it is a signed replica, otherwise it would be in a landfill.