Please see the following post by a reputable poster (eclypse) who has the Kenwood 6070. I asked him to compare THX-EX with DD-EX during the same passages in a DVD. His reply is excellent, and honest. To me, there's NO reason to upgrade (from a sonic/FX perspective) unless it makes you feel better to see THX on screened onto the face of your receiver.
Something to know about that review is he compared the different modes on the same reciever. I've been comparing the VR-6060 and VR-6070. The VR-6070 has "active speaker equalization", "Bass managment" ect to boot! This may improve sound no matter what mode you listen to.
But then I don't have one yet so you may want to go more with what these guy say. Just food for though!
The 6070's THX gives you a sixth active speaker in DD 5.1, which is nice. And the 6070 has pre-outs for all 8 channels, aside from any possible benefits of THX. Add a stereo amp for 7.1, with delays adjustable separately for the two back surrounds.
They're not identical, as you would see if you looked at my post just above. However, if there is no need for pre-outs, and if you decide the THX filters are not useful, then the 6060 and 6070 may well be functionally equivalent. They both have "equalization" (whatever that is, exactly) and so far as I know do bass management the same way.