Sizes between Suomy and Arai seem (at least to me) to be about the same. The thing about Arai that I really like is that they come in different shapes, Round Oval, Intermediate Oval & Long Oval. A Corsair Arai (intermediate oval) for example has a similar shape to a Suomy Spec 1R whereas an Arai Profile is more narrow side to side and much longer front to back... My Suomy hat is really nice but I do end up with a slight pressure point on my forehead... never have that problem with the long-oval Arais.
As stated above, if you get the chance, try on several different brands to find the best fit for your noggin. If possible, work with an experienced helmet fitter at a show or a good shop. You will not be disappointed.