I would stick to the Bmi lounge.
As other posters pointed out, you would have to go through security at the A gates, and walk quite some time before getting to the lounge area. Then you would have to go back, go through customs and security in the B gates. It's just too time consuming and IMHO, the Bmi lounge, is a bit nicer, although it's generally more crowded than the SAS lounge.