If you plan to go to Grand Cayman, Lighthouse Point has AL 100 tanks ... or at least they did in Dec 2013.
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Not directly answering your question, but somewhat related. I recently purchased a steel HP 133 for similar reasons you're looking at AL 100: worried about being the "weak link" on local dives, limited selection of Nitrox tanks and even more limited selection of DIN valves (I use Nitrox and have DIN regulator.) All combined, it made a lot of sense to get my own tank.
As for the size, 133 is the first size where they increase diameter and decrease length, so it's just an inch or so longer and equally fatter than an AL 80. Due to increase in diameter, it's quite a bit shorter than the HP 120. Also, if it gets filled to 3,000 psi it still holds quite a bit of air. Even a "hot fill" on boats that results in cold pressure of around 2700 psi gives you over 100 cu. ft. with HP 133.
From my rental experience, anything but AL 80 yoke as a rental is a gamble.
AL100 are not common in the Caribbean. Many shops have 1 or 2 AL100 but they end up being closer to 90's as they are short filled to 3000psi not 3300psi.
Not quite; an HP 117 is fatter, and either just shorter, or about the same length as an AL80.
I learned a great tip from a former navy diver. I sat watching TV with my mask and snorkel on. First I lasted about 20 minutes, but as I learned to relax I soon was up to watching full length movies with my mask on breathing thru my snorkel. It sounds funny, looks weird but holy cow did my SAC drop nearly in half. Where I use to get about 30 minutes out of an AL80 at 80' I nearly get an hour out of an AL63 now (I am still a big guy).