I have a steel SCUBA HP100cf (12.9L 3442PSI), small FABER brand steel SCUBA HP15 (2L 3442PSI), and also two carbon fiber 4500PSI tanks 6.8L and 9L. Aluminum is the worst choice due to 3000PSI fill pressure and I don't think it should be considered when high pressure steel is an option. The HP steel tanks can give you a lot of fills if you are shooting a small caliber PCP with low fill pressure. I bought the HP100 tank first and only had a 22 marauder with a fill pressure in the 2800-2500psi range at that time. I good number of fills off that steel tank, but it was such a heavy beast I didn't even want to carry it the short distance to the backyard. Cost breakdown was tank $170 (used), fill kit $150, tank hydro $40 = ~$360. The best steel option I believe is the HP80cf (10.2L) size which is the best balance of size/weight/cost. This is probably the most common steel scuba tank these days.
Now while you can get by with steel tanks if your gun and shooting style match (small calliber, low fill pressure, not a ton of shooting, not traveling with the tank much), they are still not the best option. 1)Portability - even the biggest 9L carbon tank is going to weigh half of a steel HP80 tank. You won't think twice about grabbing the carbon tank, but you will with the steel tank, and you will grow to hate the weight. Also, during transport the steel tank needs to be secured more since it is twice the weight and will be a more deadly projectile in any accident. 2) Fills - if you can only fill a carbon tank to ~3500psi due to dive shop limitations, it is going to offer similar gun fills to a steel tank, but will offer much better portability. If you can fill the carbon tank to full 4500psi, there is no contest and the carbon tank will offer many more fills than the ~3500psi steel tanks. If at any point in the future you get into a PCP with higher fill pressure or bigger caliber that uses more air, a steel tank is going to be almost useless. Exception being if you wanted to tether directly to a gun.
In short, a carbon tank setup is going to cost around double what you could get a steel scuba tank setup for, but offer a lot more long term advantages. My steel tanks now just sit in my garage since I don't shoot the marauder any more and my Impacts fill to 3600psi.