We are debating the same thing. I have a 15k btu ducted AC and a spot for a second in our bedroom. Though a 13k btu will be good for the bedroom, we are looking to move the current one forward and putting in a heat-pump so it can utilize the ceiling ducts. With that being said, as mentioned, heat is non-existent as you approach freezing. With that being said, you should be using the ducted heat for your belly tanks. For us though, with it most always upper forties to lower fifties at night, I think a heat-pump would be ideal for us. I am torn with the 15k btu up front....way too big for such a small space....
Cale