Hi Martin,
Whilst I am in no way an expert on the BL 5 and BL 9 speakers, I would like to offer the following as it seems relevant to my evening.
Coincidently, this evening I was just at a client's home which I designed about a year ago, who has just recently acquired their BL9 we discussed. Their living room is similar dimensions to yours in terms of floor area. The BL9 sound wonderful, rich and clear with a deep base note. I also think that they are the perfect size aesthetically for this size space.
Compared to this, another client has the same setup as yours (BL1 / BL2), and while it sounds amazing also, I feel the bass is not as deep and rich as the BL9 in my opinion. This however, is in a much larger space.
BL5 is of course is considered the best, although I have no direct experience (performance wise) in an actual installation outside of a showroom. But my personal opinion firstly would be they are aesthetically little large for your room, and I' m sure some of our more experience members can advise on their performance in a similar space.
All being said I'm sure you would not regret either purchase.
Good luck with it.
Laurence.