You can't go wrong with either the BCD-1 or the BDA-1 so either choice will be just fine. In my case, faced with a similar decision I chose the BDA-1 because my priority was to gain the capability to play computer generated audio and I believed that my existing CDP would improve by running the digital outputs into the BDA. When I installed the BDA-1 into my system I connected a 1 year old NAD CDP and a 15 year old Denon CD Changer to the BDA-1. The result was by far the best single upgrade I have made in 45 years of box changing and tweaking. The music just popped from my speakers with both CDPs. Musical details were there that I had not heard before -- big improvements in clarity and bass.
So given that your Meridian 506 is a much better unit than my CDPs, I would recommend you go with the BDA-1. Both of us may get the BCD-1 somewhere down the road.
Tony