I'm no expert, but doubt that fiberglass would be an advantage, given that quality metal tanks have linings to keep the water from ever touching metal. With most of these tanks, the water is in contact only with a butyl (if a quality tank) or vinyl bladder or diaphram. Of course, if you install an acid neutralizer upstream of the tank, then acid should not be an issue in any event.
I just finished diagnosing a storage tank problem at my house (flotec sourced Sears captive air tank). I would steer you away from a flotec model toward a higher-quality tank. The Amtrols have a seven year warranty, and should last 1.5-2 times that (or more, in your low-use application). The other brand that I read a lot of good things about was Flexon (though they seem to be harder to find).
From my research, the proper sizing of a tank depends mostly on the flow rate of the pump. To determine this, you need two pieces of data: (i) how many seconds the pump runs from the time it kicks in to the time it stops; and (ii) the number of gallons (incl. partial gallons) it pumps during that time. For me, it was easiest to get these data in two steps. I took a one-gallon pail to my laundry sink, let the water run (fast, then slow) until the pump kicked in (light flickers when this happens), quickly close the faucet, and then start filling up pails and dumping them until the pump kicks in again (flicker). I did this a couple of times and took an average. Then I put away the pail and got out a clock with a second hand, and slowly drained the system until the pump kicked in (flicker), and counted the seconds until the pump stopped (flicker; click). I did this a couple of times and took an average. Number of gals divided by number of seconds times 60 = GPM "flow rate" of your pump.
Next, calculate tank size by multiplying the flow rate by between 1 and 2 (assuming you have a 1/2 or 3/4 hp pump, as most homes do). The principle is that the pump wants at least a minute to cool between cycles, for best service life. The longer, the better (1 minute is a minimum recommendation). For example, if you have a 10 gpm flow rate pump like mine, you want at least 10 gals "draw down". The draw down is the number of gallons the system/tank will put out until the pump has to kick in again.
Draw down is determined by (i) the size of the tank, (ii) the cut in/cut out pressures set by the switch, and (iii) some more esoteric factors such as elevation, etc. I found a factor on the web that told me that with a typical 40/60 cut-in/cut-out pressure such as mine, the draw down will be approximately .26 X "total volume" of the tank (expressed in gallons). Total volume is the total physical volume of a tank (the water side and the air side of a bladder/membrane tank; IGNORE the "equivalent" figures published by some web sites, which just confuses things to no end). So, a 44 gal tank will have a drawdown of 44 X .26 = 11.44 gallons. The pump will run a bit more than one minute each time the switch calls for more pressure, and will get at least a minute rest between runs (assuming not more than 10 gpm draw at all the faucets at one time).
Tank manufacturers also publish drawdown for their tanks. Amtrol's figures seem generous; perhaps they have designed their tanks to yield slightly more drawdown per total volume than other tanks. More drawdown is nothing but good for the pump and for your electrical bill -- it means the pump cycles less. The only disadvantages to a bigger tank are: extra up-front cost; slightly more space requirement; the extra weight for whoever has to carry it to the basement/garage/etc. Another advantage of a larger tank is that the fewer cycles (theoretically) will result over time in less flexing of the diaphram, so that the tank should last longer. I do not know of any empirical data to back this up, but it makes intuitive sense to me.
There is a whole other debate (kind of exhausting, actually) about using CSVs (Cycle Stop Valves, a proprietary pressure regulator) and smaller (e.g., 4.4 gallon) captive air tanks, rather than the traditional layout described above. The moderator of this forum (valveman) is the inventor of this device and has a web site dedicated to it, and extremely knowledgeable. If you have a heat pump and/or do lots of irrigation, you might find this to be a better setup, and the time to make that decision would be before you replace the tank. For me (2 people; 1.5 bath; no irrigation or heat pump; renting) I preferred the lower electrical usage of the traditional set-up, and have asked the landlord to install a 40+ gallon Amtrol or Flexon tank. Good luck!
p.s.: You probably want to nail down any modifications to your set-up before installing the new tank (e.g., acid neutralizer; chlorine-injection pump), so that those fittings can be plumbed in at the same time. Moreover, a union installed between the "T" and the tank would make future service/change of the tank much easier.