Takeshi,
I'm very familiar with Rob Allen's. I've been using them since I started freediving (6 years ago) and own 7 of them (70cm - 130cm). I live in southeast Florida (very much like Caribbean & probably Mexico too). My main gun is a 120cm reel gun. I do shoot my 110cm (also reel gun) every once in a while when the water gets dirty (15-20 foot vis) and I use my 130cm (float line gun) in deeper water (70-90 ft), around wrecks with big fish, and in the bluewater. For me, if I could only have one RA I would probably choose the 120. I think for you maybe a 110 would be best or possibly a 120. Since you already own a 90cm gun, I don't think I'd bother with a 100.
BTW, I shoot single 20mm bands on all my guns, I use 7mm double notched shafts (helps with loading) on my 110cm sized guns and up and 6.5mm single notched shafts on my smaller guns (70 and 90).
Hope this helps.
Scott