A/C: better to set on run or auto

[gettingcolder]

Last August we had central air installed in our attic, running down to the 2nd and 1st floors. The unit is a Trane xl14i, with a variable speed handler. If we are at home during hot days will we be more comfortable setting the thermostat to run rather than auto? If so, what is the difference in energy consumption.
If it makes a difference, I think that the installer went oversized at 3.5 tons. Other bids had called for 3 tons, but the installer convinced me that 3.5 was better (said that calcs were on the border, though he never showed them to me). Last August, I thought the a/c was running short cycles, and attributed this to being oversized.

[firecontrol]

Pros and Cons on both sides of this one.

If you set the fan to run all the time you get two things:
Pro? First of all you will more than likely accomplish averaging the temperatures throughout the house. IE: If you get temperature differences between different areas of the home during the off period and they are big enough to make you uncomfortable, then running the fan in the "on" position will help to eliminate this somewhat.
Con? The second thing that running the fan all of the time with the equipment being in the attic is the possibility that the air the unit will distribute to the home will be warmed up by temperatures in the attic and will actuall make the unit run more to cool this extra heat load down and thus will cost you more in your energy bills.