I agree that going big is not ideal. An AL 80 is overkill as a strictly deco bottle, although some divers like them for the ability to use one bottle on the whole trip, where a 40 may need to be filled every 2-3 dives.
Personally, in a cave environment, I prefer a 30-45 sized tank for deco - it is easier to get into and out of caves where there may be a high flow entrance, there is no confusing it with an AL 80 or AL 72 stage bottle, and it is a lot less bulky when stacked with a stage bottle on the way in or out. Having lots of really large bottles apparently looks cool and studdly but it is in reality just a pain if you don't have a real reason to do it.
Technical diving is not something where gearing up for the deepest dive you ever plan to make
really works all that well on the other 90% of the dives you do and if you start diving deep enough where you need a 77 cu ft deco bottle, the cost of the AL 80 is pocket change.
In most north Florida caves, O2 is the primary deco gas and unless you go crazy with the bottom times, a 40 or 45 will provide enough for at least 2 and sometimes 3 dives with a 50% reserve on the last dive.
I am just now getting rid of my AL 30's as my deco profiles have gotten to the point where they only have enough capacity for one dive with an adequate reserve, but they served me well for years and are light and compact, so if you anticipate only short deco in the 20 minute range on O2 an AL 30 is not a bad option either and they will help you keep the total weight carried down a bit.
I have a few AL80's rigged as stages, and with 32% they work well in N Fl caves as stages, and also work well as travel gas and deco bottles on trimix dives in the 200' range. With bottom mix, they can also serve as a stage on a trimix dive (but are annoyingly floaty when empty), and obviously as noted above they could hold enough O2 for a few days of deco diving, so they are very versatile. But they are also my least used tank as I prefer the LP 45's for deco and the the AL72 for a stage or as a buddy/bailout bottle as it swims and handles better than an AL 80. The AL 72 is becoming popular with rebreather divers who need more bailout gas than a 40 or 45 allows, but where carrying an AL 80 is a PITA.
An AL 80 also puts you at a disadvantage on a deeper / longer deco dive if it is your only deco bottle from two points of view:
1. In an ocean dive if you have significant deco it makes sense to get on a deco gas earlier 50% at 70 ft as a first deco gas rather than waiting to get to 20' to go on O2. 50% is an ok compromise if you only have a single gas, but it makes more sense to split the gas into two 40's, one with 50% and the other with 100% on most deeper / longer deco dives with nitrox. Using a much larger AL 80 for the first deco gas with O2 percentages in 30-36% range at an initial stop of 120-130 ft begins to make sense when you finally get into trimix dives, but is mostly overkill until then.
2. From a lost gas planning point of view, two deco bottles make more sense as the unavailability of either still leaves you one deco gas and your back gas, rather than just your back gas to complete the deco. And it leaves you with two deco regs so that potentially you could move the reg from once deco gas to the other as you ascend in the event one of the deco regs fails - although I'd only be tempted to do that in the even the reg on the higher percentage deco gas failed. Flooding the high percentage reg to access the deeper deco gas, could leave you with no deco gas, so a lost gas contingency plan makes more sense if the deep reg fails.
