Hi!
I've been a Firmie for a long time and to be honest I'd go with the Transfirmer instead of the Wave. The workouts are much tougher, there is more variety and overall give a good moderate workout.
I have the Wave and just haven't gotten into it. As a piece of equipment, it's OK but the videos just don't do it for me. They are really short and because of the Wave don't use heavy enough weights for me to feel like a got a good sculpting w/o. Plus every time I use it my ankles feel strange. The overall concept of the Wave rocking is great, I just don't think the videos are up to par with what I would have wanted, and I bought the deluxe version.
So, from someone who has been Firming for a long time, and has over 100 Firm videos....I'd say the Transfirmer workout system is better. But that's just me.
Good luck in whatever your sister goes with. Oh and tell her DO NOT order through Gaiam directly. They take forever and a day to ship/receive and their customer service sucks. Go through Amazon, QVC (for the Wave I know), or collagevideo.
Amy