I have traditionally (for the past 10 years) preferred LP steels for the following reasons:
1) I need no weight belt (single 120, double 108s, even single or double 72s)
2) I don’t pay 50% extra for the HP fill ($9 vs $6 for LP fill)
3) I have as much or more air than with AL80, even at LP (much more with overfill)
4) A slight overfill (3000psi) is still a cheaper LP fill
5) Long life of tank (I have several Steel 72s from the early 1970s, and they all easily pass hydro every time)
6) Cheap to buy used - I’ve bought all mine used for $50 for a 72 to $150 for a 120. I just don’t trust a used AL80 to not have been abused.
However, lately I have branched out to a wider variety of tanks for different uses:
1) Double AL80s for warm water in a wetsuit
2) Double HP80s (from the 1990s) for their short height, small diameter, and medium weight
3) Double steel 72s a really cheap set of doubles, smaller & less buoyant than AL80s
4) Double LP108s for cold water in drysuit and those long deep dives
5) Two AL50s side mounted under arms (for shallow diving in wet suit) Very maneuverable.
6) Two steel 72s side mounted for longer or deeper dives still with excellent maneuverability.
7) Single AL80 or single AL30. These are really only good as stage or deco bottles along with one of the primary setups above.
8) Single LP120 with Y valve for medium dives in cold water with dry suit. Y valve allows use of doubles regulator setup on this single tank. Same buoyancy characteristics as the double LP108s with less total weight and less total volume.
I was a little hesitant about the HP80s, but for $150 including 2 tanks, manifold, & bands, I couldn’t pass it up. We’ll see how I like paying $18 for 160 cu ft of air when I’m used to getting 216 cu ft or more for $12 in the LP108s. The HP80s are really comfortable to wear, though because they are so short and skinny.
My recommendation to anyone is to find some nice old steel 72s and buy a half dozen or so for the cost of one new HP steel tank or 2 AL80s. Then you have enough of them to make 2 sets of doubles or 2 side mount setups plus a stage and a deco bottle or 2 single tank setups. I’ve found that just a slight overfill of these (to about 2500psi in the 2250psi-rated tank) gives you as much air as an AL80 at 3000psi in a smaller package with better buoyancy characteristics.
Mark