It's the 'law of diminishing returns' coming into play, and there is no easy answer to your question. It is virtually impossible to compare the sets side by side with the same signal playing through them so you can only read as many reviews of both (and other) screens from magazines and the internet, and then go and have a look at the screens in question.
Like with HiFi the more you budget the less the perceived improvement. A £100 cd player should sound a lot better than a £50 one, but whether a £2000 player is that much better than one costing £1000 is more open to debate. Sometimes it is better to be poor than rich!!
If at the end of the day YOU cannot determine which has got the better picture and, after factoring in connections, looks(personally I prefer the 'look' of the Panny TH-42PX70 over its' sister PZ70) and maybe sound, then buy the one you want regardless of budget. If your budget was £1500 and you go with the Pioneer, then fine. If you go with the Panny then you have a 'bonus' £500 to spend or save. And if, after buying the PZ70 you cannot decide what to spend that £500 on you can send it to me!!!