My wife and I live in a mountain valley in Colorado. We are at just about 8000' above sea level. The hills rise up behind us and in front of us. In winter, because of the mountains behind us, we get sun from about 10 AM to about 2:30 PM. It is cold. We have had snow on our roof since middle October. Every morning for the last week and a half, the temperature has been below zero - not as cold as some other places in Colorado, but still cold. We have a fireplace insert (brand is Lopi, but there are lots of others equally good or better).
We have a 2700 square foot house built in 1948. However, we have taken pains to insulate it very well For example, our attic has a rating of R60, and we had a thin layer of foam sprayed to reduce exfiltration. With all of that, our woodstove easily heats the entire house in the coldest weather.
I agree with s219 above, in that a free standing stove is better than a fireplace insert, as it is out in the room. However, the difference between a free standing stove and an insert is much, much smaller than the difference between an insert and a fireplace. I love the ambience of a fireplace but - for heat - go with an insert.
As jk96 suggests, go with an outside air intake if you can. We couldn't, but the heat output of the insert far overrides the inside air loss. Still, I would prefer an outside air intake if I could do it.