My biggest complaint against the 665 are the contraints on inputs. Without assignable inputs, it is far more restrictive in terms of what can be connected to it than the 663.
It comes down to weighing the features and price of both receivers and deciding which is the better choice. For most people that I know, the 663 is the better choice. If the 665 is better, than stick with it.
In the end, though, don't assume that something is better because it's newer. A number of receiver lines this year seem to have gone downhill in terms of features and performance, and I put Yamaha's 600 series in that group.