"Can anyone suggest which should be a better buy?"
A coin toss.
The question, does one buy the fuller featured unit comprising of last year's technology or do they go with a step-down in features by going with the latest and greatest gee-wiz tech-no advancements? The best answer will come from you as that's an ego question one must answer for themselves. I purchased the SR5007. Why? Mostly price as my ego wanted (needed) the SR6007 but my wallet could only rationalize the SR5007 when compared to either the Denon AVR2313CI or the AVR1713.
I wanted the latest and greatest in gee-wiz technology (future proofing) and did so at the expense of features I likely wasn't going to use when compared to suggested alternatives.
"Also ... one more question - noticed that neither of these receivers support THX format. How imp that is?"
Go to the THX website as THX is a certified, achievable standard. IIRC, my understanding of THX is, it's a reference standard of driving the room to a reference standard of 85dB with a +20dB headroom and the ability for the system to match and hold this standard. And, of course, a willingness to pay a licensing fee to attach the THX logo to your system appliance and accompaniments.
Once again, when one discusses a question such as yours surrounding THX, the importance is to your ego as in my opinion most half-way decent home systems can meet the THX standard. Depending on room size, speakers used and the inclusion of a 12" powered sub-woofer, I'd say either the SR6006 or the SR5007 can meet the licensing standards of THX for home theater reproduction.
Now it's your turn to flip a coin and call it.
Hope the above helps.