If there's one topic you'll find on many motorcycle discussion forums, it's what helmet to buy, Arai or Shoei. Both brands are top notch, high quality and both helmet manufacturers make similar type of helmets.
Comparing both helmets is like comparing two types of apples. The color may be different, the shape might be slightly different, but both are apples, and taste like apples. But what sets them apart?
The biggest difference between both brands is the shell shape. This is the most critical selection criteria you have, since your head must fit snug-as-a-bug inside the helmet. Since no head is alike (apart from maybe identical twins), you really need to try it on. If you can't fit it on properly, and if doesn't sit comfortably, the helmet is not for you.
So you first selection criteria is the fit. If one does not fit properly, your choice is made. If both fit properly, then your dilemma just got a bit bigger.
Both helmets are quality, and therefore expensive. Your life depends on it, so let's not try to save money at this stage. Both manufacturers have prices quite close to each other, so don't use price as a criteria. It's the last selection criteria, if everything else fails.
Next look at the weight of the helmet. Chances are you'll be wearing the helmet for hours, so better make sure it's light.
Finally, look at the extras. Does one have a sun-visor, the other not? How many air vents are there?
As you can see, there's no real winner in the Arai versus Shoei debate. Both are great helmets, both offer many features and models. But whatever your choice, you'll be happy with it for many years, there's little doubt about that.