I would still want to know what the transition point is from stage 1 to stage 2.  It will probably do a better job of dehumidification in the shoulder season (which may be what he means), but maybe a bit "less good" on a design day because of being a bit oversize, although a 1/2 ton difference is small.  Can you tell us what the cost differential is?

We use UV lights in all of our commercial air handling units (many millions of sf).  They work.
P.S. If you keep your coils clean and have a well drained SS drain pan, you probably don't need it the UV.  The potential IAQ liability of not having them drives many decisions.

Bruce

If the 2-stage version is only jumping from 3 to 4 tons, and Manual-J says the design day load is 3.5 tons, odds are pretty good that a single stage 3 ton would be as-comfortable and probably still keep up (especially if your house is tight and has some thermal mass to it, like a slab foundation.) At 3 tons it would only be ~15% undersized for a 3.5 ton load, and with the error margins built into the calc, probably not undersized at all.

From both an efficiency & comfort point of view it's better to be 15% undersized than 15% oversized, unless you're in the habit of leaving it off for long periods then playing catch-up from a large temperature overshoot.

If the 2-stage is making a jump from 2 or 2.5 tons to 4 it might be more comfortable though, since it would run almost continuously during warmer weather, and would the capacity to recover more quickly if you run the house at different temps at different times of day. I have no idea what the typical difference between lo & hi is on 2-stage AC, but step from 3 to 4 tons just doesn't seem that significant to me, despite being a 33% boost. Most of the cooling season hours would typically you'd be at half the peak load or less, so a 25% performance from peak AC would do something for efficiency, but not nearly as much a 50% reduction.

Bruce, the difference in price from the single stage to two stage is 2000 dollars, but right now Rheem is giving a 400 dollar rebate, blah blah blah...

I think I'll save the extra cost and get an air cleaner and UV light with it instead.

Thanks everybody for your continued help.