In my opinion, it's little more than a marketing tool at this point. There are also other issues involved that sometimes prevents something from being certified. For instance, it costs money to do so and many lower end models aren't certified because the target market doesn't care about the cert. Additionally, Yamaha doesn't certify anything because they won't give LucasArts access to their DSP technology (which is incredibly impressive).
If both carry the features you want, throw both on a credit card, put them in your system, listen to both and return the one that doesn't sound as good as the other.
Lastly, both do DD EX and DTS ES, superior to THX EX to me, though the Denon also offers Neo:6 which is pretty swank as well.
