For the two Denon models and two Marantz models, all have 9 channels of amplification and 11.4 channels of processing. Some of the advantages with the more expensive models are:
- Approx 20% more power (though who knows what the percentage increase would be with the Denon regarding multiple channels driven)
- Component video and composite video inputs (useful if you want to connect older components)
- 7 vs 6 HDMI inputs
The Cinema 40 is also a more premium build.
For myself, if choosing between the Cinema 40 and 50, I'd go with the 40. But that's because I need the component video input and I'd really like to have 7 HDMI inputs as I currently have 5 sources and plan to increase that.
So you'd have to see if there are any features that you'd need in the more expensive models and then figure out if the increase in cost would be worth it.