They used to be the same models on the inside, but not anymore. Audioholics did a comparison and came up with this:
What Makes a Marantz Different Than a Denon?
To start with, Denon and Marantz have difference engineering teams responsible for the final "house sound" of each brand. On the outside of each receiver, we can easily see that the front panels look different, the user interface is slightly modified, and Marantz tends to have higher quality binding posts and legacy audio support. But, what about differences we can’t readily see? What can you see if you crack open receivers from both companies? Below is a brief list of some, though not all, of the key differences between the two lineups. Note that these points are specific to the 08' Marantz receivers versus the 2013 Denon X series. Also note that some of these differences only apply to certain receivers in the lineups.
Current feedback preamp topology – Allows for higher slew rate and lower distortion at all volume levels (particularly at low levels) than comparable voltage feedback designs
Decoupled capacitors - To reduce residual noise and interference
Luxury power capacitors – Custom capacitors, larger values for Marantz SR7008 compared to the Denon AVR-X4000
Hyper Dynamic Amplifier Modules (HDAM’s) – A discrete preamplifier circuit module proprietary to Marantz that goes between the volume control and power amplifier section to further improve performance.
I think Marantz/Denon models used to be close, but that is no longer the case.