OK let me be more precise.
They DO use the exact same decoder, and that decoder is part of the same chipset also used by the previous generation machines, and none of them decode DTS-ES or 96/24. If you think different, press display twice and take a screenshot of the audio stream being decoded. Either Queen Greatest Hits DVD or a LOTR Extended DVD should supply popular source material.
Does the LX52 sound better than the 320? Well that would depend on exactly how you have them hooked up and to what. 2ch Analogue? Quite possibly, LPCM over HDMI? If you're using a new Pioneer Multi-ch PQLS amp then quite possibly too. So yes, you should listen for you yourself, but also understand where you are most likely to see a difference as if the PQLS thing makes a difference and you were unaware of it, didn't test it, and then bought the machine without it, you may have made your choice under the wrong impression. And if decoding DTS-ES or 96/24 is important in the player, and not the amp, then take a disc with one of them on it just to double check.
So I am not trying to say they will sound the same, just that differences lie outside the decoders, and should the OP ever plan to get a new Pioneer amp to match later, they might want to consider the real differences up front, which may leave more possibilities for upgrade later as I doubt Pioneer will drop PQLS any time soon. And seeing as LOTR hits Blu-ray later this year, and it's a hugely popular set of films, I suspect many of us may be tempted to try the interactive extras for a change which will mean in-player decoding even for HDMI, and so things like PQLS start to take on a greater significance on improving the quality of player decoded audio.
And I use the word possibly because sound can be highly subjective and down to personal preference. Some people hear differences and others stand there with a bemused look as they can't hear it. So it's very difficult to say you will definitely hear a difference, or even if you do, that you will prefer the same one. I apologise for trying to help. At least I didn't recommend the O......