If it's your first Curls product you might want to get the
Milkshake because it has so many HG reviews! I think a lot of people have problems getting Spirals to work for them, and I don't see as many reviews around for the Souffle either, although both do have their followers (I like Sprials, haven't tried
Milkshake or Souffle yet).
I'd say
Milkshake is the safe option for a first purchase.