There probably is no single "best" way to carry concealed, even in Hawaii (were CC possible for nonLEOs, of course) with its always-warm climate.
IWB does offer a lower profile than OWB, but there are many OWB holsters that are concealable enough under only a cover shirt. I use both ways, and sometimes a Smart-Carry, which offers the maximum concealment possible.
My preferred IWB is the CompTac C-Tac, a kydex holster with precisely adjustable force for retaining the pistol in the holster. I own an M-Tac too, but it rests undisturbed in my holster box because I found that because the inner surface is leather, the retention force can not be adjusted and is way too tight for me.