It depends on your goals. If you are only going to be doing pressure casting then you don't need a super powerful compressor, 2 CFM should be fine. If you want to do vacuum then you should get at least 4 CFM if you plan to use the compressor to drive a venturi vacuum pump like the HF one.
Since you are just pressurizing a vessel as opposed to running air tools, you do not need a big accumulator (tank) on the compressor. A 1 gallon accumulator at 120 PSI will charge a 2 gallon pot to 60 PSI so there is little need for huge accumulators in your application. A gallon or two should be fine. Once again if you are going to use the venturi vacuum pump, a slightly larger accumulator is nice, 2 to 4 gallons would be better.
Aside from that, the oiled ones will be a bit quieter then oil-less for the same capacity, but will require occasional oil changes. Frome a durability perspective, if you are just using the compressor occasionally for casting either will last quite a while. If you are running the compressor for hours a day, the oiled will last longer.
