I can give you my opinion and what I did.
In house #1 I went with what they told me and was sorry.Every time I had a party the air couldn't keep up and would get quite warm inside and the furnace would shut off and come right back on if it was cold out.
House #2 I went with an air conditioner 1/2 ton larger and furnace next size larger and they both worked out great.They would run long enough but not to long.I was very happy with going a little larger.