If my stated goal is to replace lead with tank weight, would I be better off with a LP 95 (which is also easier on the rest of my rig)?
Thanks.
You're getting great information here.
Aside from the bouyancy differences that have been pointed out (and thus differences in the amount of lead on your belt), don't forget that the weight of the tank is in a different place on your body when you remove lead from your belt and trade for a heavier tank. This will change your trim - that can be good, and that can be bad - it depends on your specific situation. For me, diving LP steel tanks means going to an aluminum plate (with aluminum tanks I dive a steel plate) to keep my trim right, especially in doubles. Steel HPs aren't as bad for me - but both put my CofG further away from my back, which is to say that trim becomes more "finicky" than when I'm using aluminum tanks.
Also, +1 on the fact that some shops fills ALL tanks to 3000 or 3300 psi as a standard procedure. Thus, in those shops, if you're having them fill an HP tank, you'll be getting a short fill and your 100 is now closer to an 80.
A really expensive 80.
For those that point out that, for example, an HP100 = 20 cuft of gas more than an AL80, I like to point out that for the price of one HP100, I can buy two AL80's, and 160 cuft of gas is definitely more than 100 cuft of gas. Of course, that usually gets their panties in a wad...
Given that most of my diving is in salt water, I've also had some corrosion issues with ALL of my tanks... But then, I dive just about every day, and for more "normal" divers, it'd probably be a long time before any issues arose. When there is corrosion, however, aluminum tanks turn hazy, then dark. The dark is aluminum oxide, which is very hard, and actually will help prevent further corrosion. Steel tanks turn an ugly brown (which always manages to get everywhere) which is iron oxide... And iron oxide is very soft, so the rate of corrosion increases.
Someone else mentioned that some galvanizing is better than others - I've found this to be true, too, and am interested in some of the epoxy coatings that some tanks are coming with today... But I've found a good solution is to dive exclusively aluminum tanks in salt water. Once a year, when I VIP my tanks, I clean them up and make them shiny again with a stainless wire brush, stainless wool, or Scotch Brite pad (depends on the level of corrosion) - simple. All of my tanks are stripped, spun finish to promote this. I have found that painted aluminum tanks suffer wherever there's a scratch (it WILL happen) and salt water leaks beneath the paint... Regardless of how well the tank is rinsed after the dive.
All of the regs I own are DIN regs - but every valve on every tank that I own is a convertible valve so that anyone can use my tanks at any time. I like Thermo's version of the convertible valve.
If you choose an HP tank, Thermo's convertible valve may not be appropriate pressure-wise - you may need a full DIN valve for that (the orifice on a full DIN valve is slightly deeper than the orifice on a convertible valve, and therefore rated to handle higher pressures).
If it were me, I'd dive AL80's exclusively in salt water and consider others only if I was diving dry in fresh water. Even then, AL80's make great stages, so you can't go wrong owning them. Being set up for AL80's also makes vacation diving easy, as they seem somewhat universal and easy to get - at least on this side of the world. If I were doing long penetrations in freshwater (i.e. caves) in a drysuit, I'd dive LP95's (or LP104's) doubled... With AL40's and AL80's as stages. If I were diving a "light" backplate, transpac, or "regular" BC that needed the trim raised onto my back, I'd consider a set of HP steels - but for the price may still opt for aluminum and just use some trim weights.
Good luck with your decision!