Pargo,
That's not an easy question to answer. It's different from reef diving in that the conditions and required range and power to push through big fish can change so much. We might be diving shallow stuff in relatively murky water shooting mangrove snapper, move a little to clean green water where we are hunting wahoo, or go a little farther and be in blue water hunting tuna.
I have 3 guns that I use all the time.
(1) 52" enclosed track rear handle gun with a reel. It has 43" band stretch. I shoot a 5/16" flopper shaft using 2 5/8" bands. 9/32" shaft would work fine, but we often see big cobia, which tend to make a mess of small shafts.
This is my dirty water gun, but it also sees action in blue water when I put down the big gun and grab the small one for hunting inside the rig. With all the rig legs, etc. it's nice to have a smaller gun inside the rig.
(2) 61" Enclosed track hybrid - The use of this gun overlaps with the little one in that I have it rigged with a reel for hunting in and around the rig legs, but it's also setup so that all I have to do is clip a float line to it for hunting bluewater pelagics. It is powered by 4 5/8" bands, shoots a 5/16" shaft with a slip tip (usually), and has 51" of band stretch. I generally grab this gun when I think there's a good chance of seeing bigger fish around a rig, or for a light bluewater gun. It is awesome on wahoo since it has good range and moves like a dream through the water. I shot a 96 pound AJ with this gun at close range. The fish was swimming toward me - I pulled the trigger when it started to quarter away. The shaft entered just behind the gill plate and exited near the anal fin on the other side - it went through about 3 feet of fish - it was a very close shot though.
(3) 65" molded carbon fiber gun - This is my tuna gun. It shoots either a 5/16 or a 11/32" shaft with a slip tip. It has 56" of band stretch and is powered by 4 5/8" bands. In making this gun I wanted all the power of BIG tuna gun, but wanted to minimize it's size and cross section to make it easier to swim with and handle in the water. This gun took a 194 pound tuna and a 94 pound wahoo this year.
I think all three guns are needed for all the different situations that we find ouselves hunting.
If I had to choose just one, it would be the 61" enclosed track hybrid.