the watt is max and not rms so it doesnt mean much.
both of them ARE upconvert dvd players so that is not a difference.
the ONLY reason i recommend the sony is because it has TWO digital audio inputs which means you can get true surround sound from other sources (cable box, video game system, etc). the panasonic has ZERO. you will get annoyed by that in the future. i get customers all the time complaining to me about it.
sound wise, i personally think the davfx500 is more clear. the panasonic can get loud with a lot of bass...but its not a clear clean sound.
the only downside about the sony would be the proprietary speaker wire connection as talked about before. a little solder will solve that problem.