The B-Comp is a stereo compressor. it was originally designed after testing diode bridge compressors. The Vintage/Modern options vary the attack and release characteristics of the processor and will vary the 'colour' of the output. Switch between them to hear the difference and select what works for you. This switch has more to do with 'colour' or 'feel' of the sound:
Note: The B-Comp does not introduce 'hiss' or 'analog noise' when in the 'vintage' mode.