few questions on dvd players

i am not very new to dvd equipment, but i have a few questions about them as far as hook ups go and what not.

1. if a dvd player has a built in decoder, would it be better to use the 5.1 outputs or still go out optical or digital coax?

2. and why get a progressive scan dvd player, if the hdtv has built in progressive scan?

3. on dvd audio, can a dvd player w/ built in decoder decode them (like a dvd w/ sacd), or is it different internal software.

....these may be odd questions but not sure on the issues, thanks, joe