They're both made by the same company, so they use many similar parts. Some guys say that Marantz is just a rebadge of the Denon models, but personally I don't think so. While many of the parts are extremely similar, to identical I can't see enough similarity looking inside to justify saying the Marant's are simply a rebadge.
Anyways, As long as you're going to be hooking up the receiver to a TV via HDMI I would go with the Marantz. I had a 5006 and thought it was an awesome receiver. Both are going to give you similar features and connectivity, but my own personal experiences with similar Denon and Marantz models had and has me leaning towards Marantz. I just think they deliver better sound, but that's only my subjective opinion.
The best thing you can do is go and try to audition and play with both models to see which you prefer. The feature set is going to be similar, but connectivity is going to be where they really differ.
The Marantz delivers 9.2 with a full range of pre-outs, plus 7.1 channel analog in.
The Denon only has 7.2 out, with only a zone 2 pre-out and no multi-channel in.
The amplifier section is similar and at a difference of only 5 watts they'll be much more similar than different. Again, try to go play with both models and see which you prefer best.