Hi,
I have a 6,000 Btu Olympic Catalytic heater, and it is a 1994 model without problems in all those years. I leave all three vents open a tiny amount, along with leaving open my over the kitchen sink window (I guess if rain blows in, it will go into the sink?)
I would suggest the larger model, and then if you need more heat, then running the furnace a minimal amount on those rare 15 - 19F days will provide the heat needed to keep it 65 - 70F inside. It also will circulate air into the basement if you have one, to keep the pipes from freezing - (hopefully!)
Yes I know I am in danger by using a un-vented heater in my RV, but they are well tested and known to not give off much CO gas. Probably less than the stove or oven does.
I hear the heater that you are looking at can re-ignight itself, so it is not a requirement to leave it on all night long, like mine is. I have to decide about 7 PM to bring it out and run it all night, or not open any vents, and try to keep in as much heat as possible from the prior day and just run the factory installed furnace.
I am sure that my furnace life is enhanced by having the Olympic heater. If I had to run my furnace all those hours, it might needed replacement a few years ago, but it is still working great.
Fred.