Having just bought the NAD C356BEE, I can say I can very happy with the improvement in sound over my Denon 1908 AV receiver. Didn't get to compare it with a C355, as these are not on sale any more in Sweden. However, I did compare the 356 to a Marantz PM7004 and Cambridge Audio 740. Both of these products had their pluses, however I went with the NAD, as it had a better and clearer sound at lower volumes, great control in the lower frequencies, and power in spades.
Compared to the 355 it has the MDC slot for upgradability and some more refinement that has trickled down from the M-Series (according to NAD). If you have both available, give them a test listen. Then it just comes down to price vs. performance