Please keep in mind the STR-DA3400 is being replaced by the STR-DA3500 now. The price of the new AVR seem to be somewhat higher in my country, but this might be an initial issue.
A one year old AVR is also one year closer to 'end of life', according to me, as you see how codec, connections, etc. evolve nowadays. I would rather go for the newest equipment in this case.
I'm having another post in the forum, since I'm suffering a bit from the same choice problems.
I think both are fairly equal, but quite some people judge Sony to be 'mass production' stuff, while Denon has bit the 'high end audio' aura. Whether this is really true in the case of these AVR's, I'm not so sure.