There are a lot of possible tank selections and while there are some odd ones that may be personal favorites, in general there are only a few that really make sense.
They are arranged below in order of both empty weight and capacity in terms of a North Florida 3600 psi cave fill.
1. Worthington X7-100s. - They weigh 33 pounds, are 24" tall, 7.25" in diameter and are -2.5 buoyant when empty. They hold 99.5 cu ft at 3442 psi and hold 104 cu ft at the same 3600 psi you normally get in north florida.
2. Worthington LP 85s - They weigh 34 pounds, are 25" tall, 7.25" in diameter and are -0.7 buoyant when empty. They hold 82.9 cu ft at 2640 psi but hold 113 cu ft with a north florida cave fill of 3600 psi.
3. Faber LP 95s - These are one of the gold standards in cave country. They weigh 37 pounds, are 24" tall, 8" in diameter and are -1.2 buoyant when empty. They hold 95 cu ft at 2640 psi but hold 130 cu ft with a north florida cave fill of 3600 psi.
4. Worthington X8-130s - They weigh 43 pounds, are 25.5" tall, 8" in diameter and are -2.0 buoyant when empty. They hold 131.4 cu ft at 3442 psi and hold 137 cu ft at the 3600 psi you get in Florida.
5. Worthington LP 108s - They weigh 46 pounds, are 26.5 tall, 8" in diameter and are -2.6 buoyant when empty. They hold 108.6 cu ft at 2640 psi but hold 148 cu ft with a north florida cave fill of 3600 psi. LP104/LP108s are the other North Florida gold standard.
There are other tanks but they are either very tall (LP 121, X7-120) or very short (X7-65, X7-80) creating potential trim issues unless you are also very tall or very short, or they are just not available (PST) or offer no real advantage over the tanks listed above (LP 77, Faber 72, Worthington LP 95, PST LP 95, X8-119), or have objectionable buoyancy traits (Faber 100, Faber 120), or are excessively heavy (basically anything ever made by Heiser). Some divers may swear by one or more of the tanks mentioned in this paragraph and have specific reasons, but they lack the general utility of the 5 listed above.
Comparisions:
If you compare the X7-100 with the LP 85 it is obvious they are similar in size, weight and cave fill volume. The LP 85 has a slight edge in volume when you can get a cave fill and the X7-100 has the edge everywhere else outside of North Florida where you can get an honest 100 cu ft without an overfill.
Similarly, the LP 95 and X8-130 offer similar cave filled capacities with the Faber LP 95 having a slight advantage in N. FL with lighter weight but slightly less volume, while the X8-130 can get 130 cu ft without an overfill everywhere else. It is worth noting here that Worthington or PST LP 95's weigh 42-43 pounds and offer no weight advantage at all over the X8-130.
The LP 108 is the heavy hitter in terms of capacity if you can get a cave fill , but if you can't it offers very little more gas than the X7-100 that weighs 13 pounds less.
Does size matter?
Not as much as you'd think. The size of your tank does not really matter if your team mates do not have equally large tanks.
Why? Because most technical divers use a thirds rule for gas management with 1/3rd being used going in, 1/3rd being used to exit and 1/3rd always being held in reserve. In addition, technical dives also match their gas so that the "third" they use is the same volume as the third used by the diver with the smallest tanks in the team. This is to ensure that any "third" in the team will be adequate to get any diver in the team back out.
So in the real world, if one diver on the team is using cave filled double LP 95s with 260 total cu ft and a third volume of 86 cu ft, that is the controlling volume for the other diver with cave filled double LP 108s with 296 cu ft total cu ft and 98 cu ft thirds. The extra 12 cu ft in the larger "third" is of no use, and is in any event only 12 cu ft more gas despite the extra 18 pounds of tank weight that must be carried. So unless your buddy also has large tanks, a large tank offers you no real advantage to offset the extra weight and bulk.
Now...that may change if you someday begin making very long multi-stage cave dives and if you manage your stages with a 1/2 plus 200 gas plan where you carry all of your reserve in your back gas. In that case, the larger LP 108's do provide a potential 296 cu ft of reserve gas as opposed to the 260 cu ft allowed in cave filled LP 95s. That extra 36 cu ft then allows for another 72 cu ft in stage gas - basically two more AL 80 stages using the 1/2 + 200 gas management strategy.
For offshore wreck diving, X7-100s and X8-130's are probably the best options given that cave fills are harder to find. The 130s offer more gas (262 cu ft compared to 199) but are heavier - a potential issue when trying to come up a boat ladder in 6 ft seas. And if you need more gas with the smaller tanks, a stage with bottom mix is a viable option.
For example, if diving X7-100s you would take an AL 80 and breath it down to 100-200 psi, then finish the dive on backgas, but keep 1/3 rd of the total of 280 cu ft (93 cu ft) in reserve, and turn the dive at 3200 psi rather than 2300 psi. giving you 93 cu ft thirds rather than the usual 66 - and more than the 87 cu ft thirds offered by the larger 130's. You end up carrying more weight with the extra stage bottle, but the difference is only about 15 pounds when you consider the lighter backgas tanks - and you only carry it on those few dives when you really need the extra gas as opposed to carrying larger and heavier 130's on all your dives.
So unless you are doing crazy deep dives near the practical limit of OC or almost always need more than 200 cu ft of backgas, really large tanks are again not offering you all that much more advantage.
Hope that helps. It is not rocket science but many divers don't really think it through and succumb to the bigger is better argument even though they rarely if ever use the extra gas they offer. You are far better off trying different tanks and finding tanks that work well for you in terms of weight and trim, are within your physical limitations, and are compatible with the tanks your buddies use.
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