After several Arai helmets I have decided they are vastly overrated. I still have a couple, but I now think that between the two, Suomy is a better buy and the same build quality. If you like their graphics, that's a plus of course.
I have had need for Arai customer support in the past, and found it non-existant. They yak a big line in all their massive marketing, but fall down on that issue. As for parts availability, I would say that is a non-issue.
Do some reading of some interesting research and reviews at webBikeWorld.
Again, not knocking Arai needlessly, I have owned both and had Arai helmets for many years. But Suomy is a very serious contender.
I would just pick which one fits best, they are different critters in that arena for sure.
My two kopeks.