Forced hot water baseboard requires a boiler, and loads of pipe. Forced hot air required a furnace and ductwork. The two systems are fundamentally different. So by default it will be more expensive to install, because the materials will be more expensive.
Forced air will be cheaper. If there is no existing ductwork, I might lean towards a ductless heat pump since they are pretty efficient at low temperatures. Believe I saw a 2 ton system online for about $800-900. Installation shouldn't cost much.
One thing to consider, when changing from radiators to a ducted system is the cost for demo and reconstruction of additional soffits or bulkheads to accomodate the new ductwork.
I would recommend you get in touch with an HVAC contractor that specializes in these systems, which may have a more educated approach than your GC.
Also are you installing AC? because if you are you need to probably run duct work for that too. In any case an existing system type is usually cheaper then a retrofit system. I would get an HVAC estimate from someone who does the work rather then a GC and use that to decide.
Get quotes on both systems. If they're close when considering adding duct work for outbound vents and cold air returns, and then removing the old boiler and its associated piping for the baseboard system, than I would lean toward keeping the boiler system. It will be quieter, and there won't be airborne dust getting all over the contents of the house. I think the house will also have a slightly higher ARV. Just my double coppers.
@Erica Nagle how old are the radiators? Older radiators are worth a lot of $$$
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