The D300 has a Crystal 24/96 DAC, so they are both 24bit players. If you get the Cambridge I recommend you use a better power cord than the cheap one that comes with it. It does not have to be anything fancy, even a standard black 14 AWG cord from the computer or hardware store would be an improvement. Much tighter bass. On AudiogoN, one of those $50 absolute cords would work. Also search the classifieds.
I have never heard the CD8B, but have read it is an improvement over the old CD8.
Have you thought about buying both and returning the one you do not prefer? In 30 days if you time it right, the credit would hit your Visa/Mastercard before you had to pay it.