I have all of her tapes and if you are planning a purchase I would recommend the Fantastic 4 DVD. You are getting the best of her work, imo.
The Totally series is definitely the "easiest" of her complexity.
The Solid Gold Series has some great moves, but alot of people gave these mixed reviews, alot of TIFTing and almost no cool downs.
Still Steppin and Still Jumpin are major parties in a box! Great music and everyone is literally just having a great time!
CIA 9801 is some earlier work, but this is probably her most complex.
The Happy Hour series is right up there with 9801 in complexity, maybe even better cued, imo.
The Heavenlies are her best bar none, great music, fantastic choreography, very complex but her backround people make everything doable. Personally, I use these as a gauge to how good her new stuff is.
Christi is considered to be less "athletic" and more "dancy" in style than Cathe. To me it just boils down to personal preferences...