There are several reasons for our choice to eliminate the baseboard system: It is noisy (not just air in the lines, but obnoxious ticking as the radiators heat up and cool down). Also, the baseboard radiators take up considerable wall space in every room. And, to add central air requires the addition of an air duct system. It seems much more space efficient and less complicatd to have one system handle heating and cooling, rather than maintaining an air system and a hydronic system.
We are indeed installing two systems. The second floor system is rather straight forward since there is plenty of attic access. As for the main floor system, the floor plan is very open and there are no interior doors (only doorways) to block airflow, so a single central return seems like it might work. How and where to install the return is the big question. One contractor suggested putting the return in the basement and cutting a hole in the wall leading to the basement stairway and letting the air travel down the stairs to the return vent. This would, in effect, make the basement a return plenum, if you will. No one has been able to tell me the pros and cons of this approach. I'm trying to get opinions on whether this is a good idea or not. It solves my space and location problems, but I don't want to screw up the airflow in the system by doing it.