All I know is, after being raised by an old and very good HVAC/Plumbing man (honorary Eng. Deg.), and knowing how I hate the heat, put in the biggest unit you can afford! It will cool the place quicker, and if well insulated, will not need to run as much. I know I don't want to wait 30-45 min. for the place to get cool.
Just a thought.....................!