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ColinCB: I don't know of any scuba shop that fills tanks to 4500psi, so I would cross the CF-wrapped tanks off the scuba shopping list.
You're missing a few key points in your comparison between HP steel tanks and AL80s...and I think you're confusing dry carry weight with buoyancy specs. You really should research tank specs when doing a fair comparison. Go to
this webpage which lists the specs for lots of different tanks. In most cases comparing HP steel tanks to AL80s, you are shifting more ballast to your tank. Compare the empty tank buoyancy numbers to see how much lead you can take off of your weight belt when going from an AL80 to let's say a HP100. You'll see it's about 6 lbs.
The physical dimensions of the tank will determine how nicely you trim out while using the tank. For instance, it might make sense for a short person to use a stubby HP80, but a tall person might trim out head-heavy in the same tank. Similarly, a tall person might trim out nicely in a tall HP120 tank, whereas a shorter diver might be foot-heavy in the same tank. Conveniently, shorter divers usually have smaller lung volumes and a better SAC/RMV rate...so they can get away with using smaller tanks.
Most HP steel tanks being sold nowadays in the U.S. come with a convertible DIN/K tank valve. This means that with the yoke insert installed, you can use your yoke regulator with it.
Something else to keep in mind is that it's possible to minimize dry carry weight by going with a HP steel tank over AL, cuft capacity remaining equal. Compare how much lead + full tank weight a diver would carry with a HP80 steel vs. AL80. You'll see that the diver using the HP80 will be carrying less overall weight before getting into the water. This might be an important consideration if you have to hike quite a distance to reach the water for a shore dive.
I don't know where you dive, but here in SoCal HP100 steels are very popular on account of their capacity, physical dimensions, and negative buoyancy when empty. Steel tanks can last a loooooong time when they are cared for properly. Buying used HP steel tanks can be a great way to save money while putting together your own set of gear. Hope this info helps...
