I think it will depend a lot on the furnishings. I'm currently looking at the possibility of living in a furnished rental for ~8 months. I'd expect a deal given that I would be a guaranteed non-vacancy for such a long time, and during low season for the area. But there is a very wide range of prices that seems to roughly correspond with the quality of the furnishings. If the place looks like a magazine showroom, you are going to attract a higher paying crowd than if it looks like the inside of the Salvation Army furniture section.
Reading the reviews, people tend to be impressed by small things. Having bikes available for the renters, for example, seems to really excited people (in a beach, bike friend area). A kitchen with decent items, rather than dull knives and rusted pots, seems to get commented on a lot in reviews as well. These things bring people back and get good reviews for your place. Likewise, taking the time to put together a binder with suggested restaurants, addresses for shops, suggested things to do, etc., costs almost nothing but makes people feel welcomed and comfortable.
I've researched both furnished and unfurnished in the very small area we are looking because we aren't sure what we will do. Very, very roughly, it seems to me that furnished go for about 15% more. This is a very popular vacation spot. VRBO/Homeaway (same company) and AirBnB should give you a good idea of what comparables in your area go for. The area I'm looking doesn't seem to have much of a market for longer term listings, since it is a vacation area, so I've not found much, but CL would be a good bet if there's a decent market for such things in your area.