It depends on your growth thoughts in 5-10 years. A galvanized 24 ga base stack will likely last 6-8 years, in 20 ga likely 2-3x that. A SS base stack will last you for several times those numbers. In either, it is best to remove the base stack , clean it and store in a dry location between seasons.
If you see your 2x4 evaporator needing an up size in 5 years or less, you may just want the galvanized choice, however, a used 2x4 with a SS stack will get a better price.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.