So. there's lots of factors here. The old skinny neck PST's are much higher quality tanks than the current Worthingtons. If you look inside of them it becomes abundantly clear, Faber tanks are also much higher quality. There are a few old manufacturers, Walter Kidde etc, but you are unlikely to find them, and certainly not in large sizes.
LP vs. HP is the big question but that needs to come second to what physical size tank you need/want.
How does one find that out? Sit on the table and reach back with your right hand to just behind your ear. Basically get your upper arm as high as it is comfortable and natural and let your forearm flop back onto it. This is where you need the tank valve. Make a fist, and preferably with a second person holding the tape measure, measure from your fist straight down to the tank you're sitting on. Subtract 3 ish inches and this is your ideal tank height. This gives you optimal tank length to where you can safely reach the valve while underwater, and comfortably sit while on land. This puts the valve much higher than you are used to in a recreational environment, but if you can't reach the valve it isn't safe to dive.
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan
XS Scuba Worthington Steel Cylinder Specifications
You can then go to that page and cross reference the measurement to the length of the common tanks. Worthingtons are not on there, but are approximately the same size as the PST X series, and I linked the XS spec sheet for you. You then need to factor in your buoyancy requirements which will help you decide if a Faber is better than a PST which is better than a worthington.
Fabers are generally the lightest of the bunch, PST HP series come next, then the X series tend to be the heaviest. You can see this when you compare the PST 104 to the OMS 108's, or the PST HP 120 to the XS x7-120. PST 104 is -3.3lbs, faber is only -1. PST HP120 is -1.3lbs while the Worthington is -2. If you're regularly diving in very cold water, the more negative ballast you will need, so it's no harm in grabbing a heavier tank. I sink like a brick, so I dive PST HP tanks most of the time.
In 120 land you have the HP120's, X7-120's, X8-119's, LP95, LP104, LP108/112, LP121's. You do not often find LP tanks from PST, so while they are there, it is uncommon to find them in large sizes except the 104's which are very popular in cave country. Height range from these is pretty wild so you have the 121 at a whopping 29 inches tall which is enormous, down to the E8-119 at 24 inches which is shorter than an AL80. Most people are too short for the HP120/x7-120 and the 121's. I'm 6'4" with a long back and the 121's are not comfortable for me to sit with. The 104's are actually about perfect with the 120's being a smidge too long for me to be happy with. I like to sit back and lean against my tanks so while the theoretical length for me is actually what the 120's come out at, 28", I prefer the slightly shorter 27" on the 104's.
Fills have already been mentioned. If your shop regularly fills HP tanks, then you will likely get most of the way to a full fill. The HP and X series 119's/120's are just over 100cf at 3000psi. The tank factor on the X8-119 is about identical to the LP95 so that is why it is on the list. At 3000, the LP95 holds just over what the X8-119 does *108cf vs 104cf*. The larger 104's and 108's are truly spectacular tanks and when pumped up to 3k, hang out around the 130cf mark.
Talk to some of the more experienced divers in your area, they may have some to let you try to figure out what you like. Beware that with the PST HP series they are 300bar din ONLY, the rest of them will likely have din valves on them if you buy used. Fabers will likely have 300bars on them and Worthingtons are usually 200bar convertibles. If you have regs already, make sure they're din, if not, make sure you buy din. My personal preference is to the PST HP tanks. I like the hot dip galvanizing vs. the finish on the Fabers, though both of them are much higher quality than the Worthingtons imho. I wouldn't scoff at Faber tanks, but given the choice, for me it's the old PSTs every time. I have LP72's and HP120's that I use all the time for sidemount from PST, and prefer the 104's to the 108's for their buoyancy and extra length when diving backmount doubles.
