I had a third party (Energy Wise) that specializes in heat load calcs for spray foam insulated homes do the calcs for my home. The calcs focused on A/C since the heating will be done with a radiant system. The calcs showed that I needed 1 ton/1175 ft2. The third party spec'd return ducts in each room and one or at most two registers per room (most were one register) with flow rates of between 60-200 cfm per register. This system is designed to run 35-45 minutes an hour. The calcs can be found here.
The HVAC contractor has reviewed the calcs, but is insisting on sizing it based on his experience (he didn't do calcs), which ends up being twice the size as the calcs. He would design it to have about 1 ton/500 ft2 of house. He would not put return ducts in each room, but would instead put one or two centrally located returns. He would design it so that the space under the doors of each room is big enough to allow the air to travel under the doors to the returns. He didn't say how many registers he would put in each room, but he did say that he likes to run 800 cfm from the registers (or per room? I'm not exactly sure) so that the homeowner can really feel the cold air coming out. His system is designed to run 15-25 minutes an hour.
The HVAC contractor's rationale for his system is as follows: (1) he has had 15 years of experience doing it this way and has never had an unhappy customer, (2) oversizing is not a problem in our area because we have low humidity (northern Utah), (3) oversizing is not any more expensive because the cost of add-ons to the smaller system (returns to each room, caulking ductwork, extra labor, etc.) is the same as larger equipment, (4) risk of undersizing is much greater than oversizing because if the system is undersized there is nothing you can do later short of tearing the house all apart to fix it, (5) both systems are equally efficient because even though his draws more energy when it is on, it is not on as long as the smaller sized system, (6) the only unhappy customer he has ever had is one that went to a third party (some local mechanical engineer) to have Manual J calcs done and then the system wouldn't cool the place on the hottest days in the summer. The HVAC contractor is convinced that if I build the home per the manual J specs, I will not have sufficient cooling capacity.
I'm an engineer myself (chemical) so I am inclined to trust the calcs done by the experts (the third party). However, I would be interested in any of your thoughts on this.