It's hard to chime in when typically one has Nero or Roxio, not both.
I originally had Roxio about 3 years ago, and switched to Nero 6 and have been very happy with it. As TimC commented, Nero and Roxio used to be the big boys, now there is a lot of freeware out there that most people consider better. Most people would also put DVDShrink, ImgBurn and RipIt4Me (to name a few) in there arsenel.
But, between the two, my vote would go to Nero.