Al tanks are lighter to carry around for a given volume of gas.
Who fed you that load of bullsh!t? Whoever did is clearly mistaken because that is 100% backwards from reality
Luxfer AL80-77.4cf of gas, empty weight of 31.4lbs
PST HP80-82cf of gas, empty weight of 28.6
essentially 3lbs lighter, and that is not including the 8lb buoyancy advantage that the HP80's have... So total weight advantage is 11lbs on land, 8lbs less lead on your spine, and you have 5cf more gas... Yup, that's a real advantage. You argue that that is a HP tank so it doesn't count, fine.
Faber LP85-85cf of gas, 31.2lbs on land, with a 4lb advantage in the water, so you get a 7cf gas advantage, and save 4lbs of lead on your harness.
Why do the Euro guys use AL tanks? The big sidemount guys there are Mexican style cave divers. HP, Steve Martin, Toddy, etc etc. No steel tanks down there, so their rigs are designed around aluminum tanks. Extra advantage? With aluminum tanks, you don't need a large wing, the Razor wing is abysmal when it is fully inflated, uncomfortable from the bungees, rises off of your back, and makes you look like you grew a turtle shell. It is perfectly fine for 4 aluminum tanks, especially in a drysuit, but it is not suitable for the heavy lift requirements of big steel tanks. They don't need the big steel tanks, because most of their caves are fairly shallow.
In Florida, sidemount was developed using the same tanks that were being used for doubles because it was convenient. LP104's from PST, LP95/108/121's from Faber, all of the HP tanks from PST then Worthington. For the reason that Mr. Archer is dead wrong about, steel tanks provide a much greater volume of gas for the same given weight.
Now, in addition to that, the "fancy" techniques that you can do with aluminum tanks can only be done with aluminum tanks because they float. Add a valve and first stage on there, and breathe them down to 1200ish psi depending on the regulator, and they are perfectly neutral. This means that tank swapping becomes feasible because you are not using the tanks as your ballast so you can hand off a tank, and wait for one to be given to you without shooting to the surface. There are pros and cons to each system, I like the feel of the Razor much better than the feel of the big Florida style rigs *Nomad XT, SMS 75/100 etc*, but it just isn't very comfortable when the wing is fully inflated, feels like you have a noose around your waist. I need big tanks for Florida caves, they're deep and long. I'm doing some diving with a buddy coming up this spring, the dive plan calls for a pair of LP121's, and a pair of stages, and we will need every bit of it. If I were to do that dive on al80's, I would need 6 tanks instead of 4. Sure it can be done, but it creates a lot of drag so I can get farther on 4 tanks than I can on 6, so I carry big steel tanks.
For small dives, I have a pair of LP45's which hold 62cf when cave filled. A pair of them weigh about as much as a single al80, I have redundancy, I have 1.5x the gas that a single AL80 would have, and I can take about 6lbs off of a weight belt with them vs a single AL80.
