In addition to what James said, the BDA-1 has more advantages -
More inputs - the BP6 will only have 1, and its coax only
Remote source switching
Selectable/defeatable upsampling - the BP6 is locked at 96/24
I think the BDA-1 uses 2 DAC chips per channel vs 1 chip in the internal card
The BP6 won't do phono and DAC internally. If you don't have a phono stage or won't need one down the road, its no big deal.
I was seriously contemplating adding to DAC card to my B60. I'd have to replace my phono stage and have to get something to convert my Apple TV's optical output to a coax output. The cost of everything would be more than the BDA-1 in the long run.
Just some thoughts.