At 50" I would definitely go plasma (I have both :eater
. Anything below 42" =LCD, above =plasma.
Plasmas are far more forgiving with crappy SD feeds, and at this size that matters. I've set up 1080p Sony bravia panels, and the PQ cannot touch my Fujitsu plasma. Don't listen to some of the regassing cr*p and the suchlike you hear.
1080p @ 50" and you would start to see the benefit over a 720p native res panel with HD material, but if you are mostly going to be using SD material (which all of us are at the mo) a 1080p panel has more scaling to do (more pixels to fill) so can look awful with some feeds. So don't get too hung up on buzz words.
At the end of the day however, budget is the key. How much you got to spend?
If money is no object then the Fujitsu P50XHA58EB will provide the best PQ anywhere hands down without an external scaler (i.e Lumagen)
@ More sensible prices:
Panasonic TH50PH10 (preferable)
Panasonic TH50PX70
Pioneer PDP-5080XD
Panasonic TH50PZ700 (1080p)
A good company is AV-Sales.co.uk, and you can get a good discount being an AV****** member, but the key is to go and demo whatever your decision.