I agree with the others, go with the 14ft (or higher) ceiling so that you can put in a 12ft overhead door. You may not need the 12ft clearance today, but someday a motorhome or a taller pickup camper combo will appear in your driveway and you will wish you did. In floor heat and a boiler is great if you plan to keep the space heated all the time but you will not be happy if the plan is to only heat it when working there as it is much slower than forced air. I just completed a 1500 sq. ft. shop with 14ft ceiling, 16 wide X 12 high door and forced air heat. I am not sure, but 16 ft ceilings and a 14 ft door might have been better. I can tell you that hanging the sheet rock on the ceiling with just the wife helping was not fun!
