HSD,
For Hawaii, I think a 110 is perfect for general reef hunting. This is just personal preference. I never knew the 10cm increments made such a big difference until I started using euros regularly.
120 or longer(sometimes use a 140) I sometimes use in deep water for dive-bombing fish - drop down with the gun fully extended in front of you & use your fins as a rudders(like in the Ayau video). Easy to shoot, accurate, and streamlined. I also used the Omer as a reef gun & it does rather well due to it's lightness, but my style of diving is more conducive to shorter lengths - you may be suited for a 120 or even a 130.
I use a 100 as my goat gun & it's probably the easiest gun to use. Enough length to aim easily & short enough to swing with comfort when those pesky buggers come in. Sometimes I need a tad bit more range, but I chalk that up to my diving/stalking inadequacies & figure I have to work on that
I'll be back in Hawaii soon & have a variety of different guns not pictured including other Omers, Picasso, & Rob Allens you can try out, so send me a Private Message & we'll meet up to go diving sometime. I'm probably selling off the 120 omer & a couple others(Rob Allens are a bit too strong for my taste), so if you like one, I could cut you a good deal.
BTW, have you seen the Epic video yet? Sterling charged my card a bit ago, but it hasn't shown up yet...