I'm in the process of getting estimates on replacing heating and cooling system. My current A/C unit is a 18 year old 3.5 ton unit and it had a hard time keeping the inside temp at 78 during the hottest parts of the past summer. A few contractors spent a great deal of time measuring and looking around, but only one has done a load calc while at my house and he said that I would need a 3.5 - 4.0 unit. He said that it was borderline (43,000 btu?) for using a 3.5 ton and would recommend a 4.0 ton unit. Would oversizing by .5 be that big of a deal?