New version of the same ski. If the 5500 was your ski last year the 5.5 is your ski this year.
Can't remember what the K2 rep said was different but I suspect that not many changes were made. It was one of the best skis in the "aspiring carver", intermediate/advanced, skier category. The shop I work in sold many of these for use in the midwest as well as western conditions. Our customers had good things to say about the ski after skiing it.