Buying the Denon X-series new has the advantage that a 3 year manufacturer's warranty is included. A4L can only provide a 1 year manufacturer's warranty.
Buying a 2015 model (like the 4200) has the advantage that it includes full support for 4K video. Previous years' models can't. (Their HDMI chipsets don't include HDCP v2.2) That might be important when you upgrade your TV.
While that is very true, in practice, I bet most people don't really benefit from paying major premium just for HDCP 2.2. There are not a lot of 4K movies, and there are inexpensive dual HDMI HDCP 2.2 players, and probably in most homes with mid range AVRs, one can't really see much of an improvement going from 1080p to 4K. Last year models such as the SR7009, X5200W, X4100W are really great buys right now thanks to the overrated HDCP 2.2.
