Question for DVD Burning Gurus?

I am getting ready to buy a DVD burner for my computer so I am a newbie at this. I am going to back up my movie collection. My question is this, do you lose much video and/or audio quality when you burn an original dual layer movie onto a compressed single layer disc in order to avoid having to use 2 discs for 1 movie? Also what software would you recommend for burning dvds? Thank you for your help.

Rick