Thanks for all of your input - tough decisions just keep getting tougher!
(DRUM ROLL PLEASE...)
I went with the 50px75U - ordered today & will have it by Tuesday.
I spent the morning cutting cardboard mock-ups of the 42 & 50 and placing them on my TV stand. Switching back and forth I could only get the feeling that the 42 wouldn't be much of a size improvement over the 34" tube (the viewing area is actually about the size of the entire front face.
The idea was...