I would look around for a used pair of either HP100s or LP85s. For the HP100s, make sure they are the ‘special permit’ 3442 PSI version, not the 3AA 3150/3500 (+rated) tanks. Those are not very common, so you probably won’t encounter them, but they are quite negative.
Someone posted that you should consider what type of fills you can get, and that is excellent advice. Most dive shops outside of cave country in FL are not going to overfill tanks. In fact, for LP85s, the stamped pressure is 2400, and you might have trouble finding a shop that will fill them to 2640 (that’s the allowable 10% overfill if they have the plus rating) because many recreational shops simply don’t know anything about steel tanks, or they have restrictive fill policies that are dumbed down to the least knowledgeable employee. Then there’s the whole issue of getting the plus rating renewed at hydro time. I had nightmares trying to find a hydro shop in TX that would do it. It was very frustrating.
Those considerations point you to getting HP100s. The Faber FX100 is a really nice tank, but I’ve never used them for sidemount. They are slightly more negative than LP85s, and the thinner profile of LP85s is supposedly very nice for SM. (I use AL80s, so I don’t know how they sit) You just need to do some research to find a local shop that will fill them correctly.
You didn’t mention what your budget is. If you want cheap and plentiful, AL80s are ok, even in cold water, for SM. You’ll have to wear more weight and you have to re-clip them during the dive (every cave diver in Mexico does this on every dive, it’s no big deal) If you can find galvanized LP72s with 3/4” valves, they are really fun tanks to dive with, usually available for $50 or less, and they are my favorite tanks for shallow dives. You just don’t get much gas unless they can be overfilled, and most shops are not going to do that. They see ‘2250’ stamped on the side of a very old tank and get pretty squeamish, in my experience.
