bback up heat

hi all im currently doing my house and ran radiant heating on every floor, the 2nd floor is mounted under the sub floor with heat transfer plates, im going to be adding central air on the 2nd floor and thought about adding some sort of forced hot air for heating, i was wondering if i should use gas or a hot water loop ?
if i use a hot water loop what size pipe do i need to run from the boiler or am i just going way over kill? i figured to have the heat in there just in case its really really col d here in nyc . i would be ablo
e to turn the forced air onto get the chill out .. what say you guys?