The px80u is more comparable to the a450, not the 400 which lacks some features. I'm not sure if there is a difference in PQ, but there may be. If it must be the 400, I would go for the px80u, merely because I am not as familiar with this model.
The real comparison is with the more common 450 model. I have seen both the 50px80u and pn50a450 in a home setting and while both are capable of very good image quality from what I have seen, I would personally give a slight edge to the panasonic. Its worth noting that both owners love their sets and say they wouldn't have chosen the other one after seeing each others.
Regarding the comments on the px80u, its a great entry level set and both it and the samsung beat out any other set in its price range by a mile. The best part is its very good blacks and contrast that beat out many more expensive sets. Yes its noise reduction circuits are apparently not the best out there but the benefits of detail-killing noise reduction are dubious at best, and completely useless on high quality sources. Not a deal-breaker by any means.
This is not a general pansonic > samsung comment, this is merely speaking about their 720p models. For the higher end sets (pz series/550/650) I often make a different recommendation as there are clear differences in PQ and they have different strengths and weaknesses which can be matched with what the viewer is looking for.
Regardless, to the OP, you really cant go wrong with either. Read some reviews and check them out in store. Although do remember torched up in-store viewing is a far from accurate representation of picture quality in the home, but at least you can actually see the set.