-Well both will be much better than the B&W CDM 9NT so no worries there but if you have the means always buy the better speaker. Futuras Is an endgame speaker IMO
-the first question I have is how do you use your subs? Do you highpass (cut the lows to the mains) if not you should be. This will play a big part is what way to go. The main difference between a lot of speakers is just the bass and if you are cutting it off anyway it matters a lot less.
-the soundstage and inner detail will most likely be stripped out a bit by the denon it will just sound like it has less scale. I also have a combination two channel home theater and speak from experience (I have had many systems with denon, marantz, b&w 703s (newer version of your speakers) and I run duel subs (picture of my system in my profile). Anyway the kicker is you probably use your denon for bass management and that will make it hard to go to a two channel pre-amp if you ever want to (you should) because you would not only need to bypass the surround through the two channel pre amp but also through an external crossover. For what is is worth I do just that. Have a two channel preamp/amp with “external” crossovers in the subs and bypass through the two channel system for home theater. Works great, best of both worlds but is a lot of wires and equipment. Also family members will never be able to turn it on again, you can decide if that is good or bad lol.