Hi,
These were the two players I considered, a few weeks ago, and I went for the Pioneer 51 two weeks ago. I'm very pleased with the player so far.
I saw both in sevenoaks connected to my tv. I can honestly say I would have been happy with either. I went for the pioneer over the denon because I prefer the design and build of the product, but in my opinion, they are both very good with blu ray and dvd.
Is there any way you can get a demo, maybe at richersounds? They're both £300 instore.
Only downside is loading times for blu ray discs. Sin city took about 5 minutes to load, others are better though, and dvds aren't a problem.
You could also wait for the new pioneer 320 if you're more patient than me. It's supposed to be as good as the 51, but with profile 2.0 for bd-live (which I don't care about) and online updates (very useful). There are some customer reviews on Amazon.com (US Version) on this player.
Hope this helps. Any questions feel free to ask.