as far as price range, availability and brand notwithstanding:
new, last year's models: 50" around $7,000, 42" around $3,000;
used last year"s models: 50" around $5,500, 42" round $3,000?
New, this year's models: 50" $11,500, 42" $4,500.
the problem seems to be availabilty if you are trying for the best deql on new units. (Dell has low prices, but is pushing the delivery farther away, now into december?) Internet dealers are in limited supply, although that is changing daily. The fujitsu 50" seems to be available and popular, the pioneer 50" has just appeared and is also much sought after (though no one has reported on it's PQ yet) everyone likes the panasonics and they seem to be available. pioneer is going to come out with a 43" set which shows promise, etc.
my guess is that today the Fujitsu 50" would be the best known quantity if you want to spend 11,500, the panasonic 42" if you want to spend 4,500, both of these being new (emphasis on today, since the pioneer is shipping this week). If you want to get the most performancel for 7,500, look at the used or last year's new pioneers.