did they do a "load caculation" on the home,or just estimate based on the square feet??....my opinion,,if you have lived in the home for at least one summer,and were satisfied with the ac performance,assuming it was working properly,there is no need to go with a larger system...if you felt that the system didnt make you comfortable on a hot day/night,,then going up 1/2 ton is a good idea.. if your 3.5 ton system is original to the house,builders often size ac systems to be just enough to meet requirements,but homeowners often feel they are a little undersized,therefore go a little bigger when replacing it.......a properly working ac system will run continuously and keep a home about 75degrees when its 98 degrees outside,with some humidity..maybe a little warmer on 2nd floor if multilevel home... its true that oversizing can cause the system to not remove humidity as well as a properly sized sytem,,but i can tell you from experience that for every homeowner that complains the ac is too big and doesnt remove humidity well enough,there are 100 homeowners that say their ac isnt big enough..ac has to be quite oversized for humidity removal to become an issue....1/2 ton oversized has more of an upside than a downside..........dan
