As to answering the OP's original question, I'll echo the suggestions to try them on for an extended period before making a purchasing decision. Everyone's noggins are shaped differently; some are quite round, others are oblong, and many are somewhere in between. Each manufacturer has a different headform that they use to build their helmets. Some, like Arai, offer different headforms in the same markets, but most use a single headform for a geographic region. There's really no substitute for wearing a helmet for 20 minutes and seeing if any hotspots or pain from headsqueeze develops.
Unfortunately it's a lot harder to find a shop with a good selection of SA-rated helmets to try on, and I've yet to find anywhere online that says which SA helmets tend to fit which headshape best.
(I'm shopping for SA helmets, too—I'm a long oval and my wife's a round, so we're a bit challenged finding helmets that fit!)