Honestly, I've never heard anyone rave about either brand. I had a Rossi many years ago and it was just OK - when I went to K2 and Ride I couldn't believe the difference.
It depends what kind of riding you do, in my experience / opinion:
Freeride: Ride, K2, Nitro, Burton (sorry, but they do make a nice board)
Freestyle: Rome, K2, Lib Tech, Gnu
Good Luck
Rossignol has better boards, but they are definitely not a high end brand.