I was interested in both of these,i auditioned the zep in the shop and it sounded excellent but i ended up buying the Denon because with the right speakers (Dali lektor 1 in my case) it sounded as good as the zep but is more versatile it has a cd player,dab radio etc.It was also easier to fit into my bedroom.
In a way, I think these two should not even be compared. I love Zeppelin, my friend has it and it is wonderful as one piece equipment!
The moment you are able to split with central unit and speakers separately, it gets into a definite league altogether and if there is no space / practicality constraints for Denon, it beats Zeppelin, any day.
It is the same with Sonos, the S5 is great containing subwoofer/2 woofers/2 tweeters, but the Zp120 50 wpc Amp with Wharfedale Diamonds 9.0 just sounds more natural - in a league of its own.