One of the things you didn't mention is where your fireplace and chimney is in the house - Is it:
1. In the middle of the house w/ living space all around it?
2. On an outside wall, but even with it, so that only the back side of the chimney is exposed to the outdoors?
3. "Tacked on" to the outside of the house, so that the back and sides are outdoors, and only the front face is exposed to the house interior envelope?
This will effect your heat flow and efficiency. Assuming they have equal size fireboxes, the stove or insert will put out the same total amount of heat, the question is where does it go. For the most part it can be either delivered directly to the room by radiation or convection, or it will be absorbed by the masonry of the fireplace. Obviously heat entering the room is no problem, the question is what happens to the heat absorbed by the masonry - will it be released back into the house, in which case it's like putting money in the bank to spend later, or will it end up warming the great outdoors, which is more like spending it on beer and floozies - fun at the time, but no long term return on investment.
In case 1, it doesn't make a great deal of difference what sort of stove you use because almost all the heat that goes into the masonry will eventually re-radiate into the living space. It might even be a better deal to heat the masonry since that will cause radiant heating into areas that aren't directly exposed to the stoves direct heat.
Case 2 is more of a compromise, some of your heat will go out the sides and be useful, but you may loose a fair bit out the back of the firebox. Probably a good idea to try and minimze the amount of heat absorbed by the masonry, but no need to be fanatic about it. An insert would work, or a free standing stove in the fireplace. It might be worth trying to insulate the back side of the fireplace box, or put a heat reflective fireback behind the stove (if code and the installation permits)
Case 3 is the classic "evil chimney" - pretty much anything absorbed by the masonry is going outdoors, so you want to do whatever you can to maximize heat sent out into the room, and minimize that absorbed by the masonry... I'd probably reccomend an insert and possibly as much non-combustible insulation lining the fireplace as possible, along with a block-off plate w/ insulation on top of it, etc.
Gooserider