Truth is there are not very many significant changes from the 475 to the 476. The 476 is simply the latest model with a few tweaks that were learned from the 475. Most likely better OS support, software and security features. Perhaps a better way to look at the two would be to identify what features you want/do not want as opposed to how they are different from eachother.
As long as the newer 476 doesnt have any deal breakers then I would go with the newer model. If you can find a deal breaker with the 476 then fall back to the 475.
Newer means faster and longer support in the HP world. My observations are that HP normally only keeps its core developers dedicated to a printer model for so long before branching them out to the next model. As time goes on each model gets less and less development focus which slows down response time to errors and whatnot. Eventually all support of a model stops once a model has gone End of Life. Models generally go EOL about 4 years after thier initial launch date. After that you will have to turn your attention to the forums for support as any issues that can be fixed will likely be circulating around here somewhere.