If you know the 225 was properly cared for and the membranes are in excellent condition, I'd go with the $3500. If you aren't 100% sure of the care and condition I'd go with the $3900.
In new condition either will be great to use but a used machine is only good if it was 100% properly cared for
Dave Klish, I recently bought 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.