We were in the same predicament as you 12 months ago but ended up staying at the Sofia Balkan because its great value if using cash and points. That said, we went to the Sense rooftop one day for lunch (just to check the hotel out) and wished we had stayed there instead. It's modern, contemporary, cool styling and felt much more energetic that the old world charm of the Sofia Balkan. There are sufficient reviews about the Sofia Balkan in their hotel thread.