Roman, looks like you need to answer those questions yourself because while the steel tank answers all four of those problems, and aluminum 100 answers exactly none.
1. LP108 holds 112cf of gas at 2640 psi, C100 holds 100 at 3300
2. C100 is -0.4lbs, lp108 is -1.0lbs when empty with valve so the half pound isn't relevant but leans steel
3. doesn't matter, the lp108's have been filled to 3800 multiple times a day for decates along with the rest of the faber line with no issues. If he's not diving in cave country, the odds of getting it filled that high is moot and 3000psi over 2640 is negligible. Most shops don't fill this high anyway, if you have an inexperience tank monkey, you'll get a fill to 3k and it'll cool off to 2800 or so, so this one shouldn't even come up for discussion. What should come up is that a C100 only holds that amount of gas at 3300psi, so you are far more likely to get a short fill on that one at 3000psi and only get 90CF of gas. This is the question that divers have to make when they buy HP steels with 3500psi service ratings because the chances of a short fill are very very high, so you have to assume you'll only get 3kpsi on your shop fills when you're doing gas planning.
4. the LP108 is actually 5lbs lighter on land than the C100 when empty, so even when the 112 is filled to 3800psi with 161cf of air in it, it is exactly the same weight as a C100 at 3300psi with 100cf of air in it. Fill the LP112 to service pressure and it is 4lbs lighter.
The only two relevant questions he should be asking
Question 5. are you OK with a rounded tank bottom and having to use a boot to keep it upright, AL tanks are square bottom so they stand up. The faber stock boots are terrible and have to be removed for cleaning when you dive in salt water to keep the tank from rusting. This is easily solved with one of the hexagonal boots which also keeps the tank from rolling around and really isn't enough of an ever so slight negative to even consider
Question 6. Cost. C100 with valve, brand new should be $250, if you pay more, you're getting ripped off and make sure it is a convertible valve, not a yoke valve. Brand new LP108 is like $470. Now, in the long run, if you spend $250 on a C100 it's a waste of $250, because the tank is useless and should never have been made. Paying full price for a tank is something I will never do because you can find a set of doubles for like $400 with a manifold, so check the classifieds here and check around craigslist. Much better life decision to buy tanks used because the price never really changes for them. Even still, the extra $200 is well worth it for the steels. I have a pair of LP72's with birthdays from the 1960's that still have their + rating and most shops won't even look at an AL tank from pre 1990. This is likely a one time issue with Luxfers alloy, but the lifespan of aluminum tanks is always going to be much less than one from a steel tank, just a fact of life. Bigger up front cost, better lifetime ROI
