Jugarf,
I used to have a larger firebox, 3+ CF, now I have a month old Olso, w/ 2 CF firebox. I have similar limitations in that I installed my Oslo on the hearth, in front of the fireplace. I needed a rear discharge lower than 27". So I could not use the Firelight and bought the Oslo with the short leg kit. I have not gotten into my 24/7 mode and only had a few fires since this November has been moderate, mainly in the 40 to 60s range. (Great heat pump weather and I save my wood for the 20 F nights to 40 F days or lower.)
The one concern that I have with my Oslo is the time I can go, either sleeping or to work, and still have coals so I do not have to start a cold stove every day. The new EPA non-cat stoves do not allow you to shut down the air to have a long slow burn like my old cat stove. So if you are going to be a 24/7 type wood burner, I would recommend the larger firebox. In another month or two, I will have been burning in the 24/7 mode and have a better feel for my stove. But right now, I am thinking that my be the main draw back. I do burn mainly red oak so I am hopeful I can handle it. I also tend to get up about 5 am, 6 hrs of sleep, and can get the stove going in the morning. But it is better if you have a bunch of coals verses a few. Also, I can be gone 10 hours during the day on average.
I also heat about 2000 SF on my first and second floors, basement not included. On a single digit night, or teens with a 20+ NW wind, the Oslo will be maxed out and my central heat, heat pump, will probably be tapping the Aux heat. So an extra 11,500 btu/hour will be very useful on those bitter winter nights. But in my region, I usually only have about 10-15 days of that weather in an average winter. So at this point, I do not regret buying the Olso, as I did not have a choice with the Firelight. But I do have some concern, and if I did have the choice, I would go with the Firelight.
I would encourage you to consider installing the stove out on the hearth, instead in the fireplace. And install insulation and a heat shield in the back of the fire place if you have an exterior masonry fireplace. With my insulated heat shield I have 170 to 200 F space behind the stove. A lot of heat that would have gone out through the bricks.
GE