Go with the one that did a Manual J calculation (which I assume none of them did or they would explain it to you).

Here is a presentation I found real quick that appears to explain it: https://www.energy.gov/sites/prod/files/2013/12/f5/webinar_hvac_calculatingloads_20110428.pdf

basically, you calculate the the loss through the walls (based upon the insulation, windows, size of the external walls), roof, etc... the area the house is in (the need for heating and cooling based upon outside temperatures), and it will calculate the number of BTU of heating and cooling you need; then you can size up a "hair" from there to account for the non-seasonal days, or when the load changes quickly.

Rules of thumb work, but it is very easy to get it wrong because they do not take into account many important factors, such as the condition of the house (insulation, etc...) or area in the country (which obviously makes a difference).