I went with the 306/200 but would be happy with either. The 306/200 has 8 24-bit Burr-Brown DAC chips (vs 2 in the 303/200) for a lower noise floor and wider dynamic range and it will do 96KHz or 192KHz upsampling. I also use my 306/200 as a DAC for various other digital sources so the extra digital inputs come in handy. Both players are outstanding but whether the 306/200 is worth the extra $2K (list), only you can decide.
