Hi,
you must distinguish:

HD is always transmitted in 16:9 so neither letterbox nor pan&scan do anything.

SD if picture is 4:3
- Just scale makes people look fat because the picture is stretched horizontally until the black bar is gone.
- pan&scan keeps aspect/ratio and people look OK because horizontal and vertical stretch is the same. You loose some information on top and bottom, it is cut.
- letterbox shows pixel 1:1 without stretching, bars left and right

SD if picture is 16:9
- Just scale makes people get egg faces because the picture is stretched vertically until the black bar is gone.
- pan&scan keeps aspect/ratio and people look OK because horizontal and vertical stretch is the same. If movie is in >16:9 like 21:9 (often blockbuster) you have small black bars on top and bottom.
- letterbox shows pixel 1:1 without stretching, bars left, right, top and bottom

I prefer pan&scan.

ciao