Amtrol makes the best tanks -- the Well-X-Trol, but it makes some lesser tanks also. The WX-202 would be the Well-X-Trol 20 gallon tank, but if you went to the WX-203, that would be better. So would you be better with a larger but lesser quality tank? I don't know. My 44 gallon lesser tank is doing well after 12 years.
Not doing a lot of lawn or garden watering I presume.
I suspect that your well is 170 to 250 feet deep. Water depth can have an effect on how much water a pump delivers. Your 5 GPM pump could be putting out 7 or even more, for example.
I had my pressure switch set for 30-50. I replaced that switch with a 40-60 when the old switch did not work quite right. I have it set for 35-55 right now. I only notice a difference for lawn watering, and I do that only after new sod or seed. A pressure tank has slightly more drawdown rating on 30-50 PSI than for 40-60 PSI.
While you are deciding, you can get a little more time out of your old tank by adding air, even if the bladder/diaphram is shot. I don't know how often. I have seen people post about this.