Step Blast is in a class of its own as a step workout. It is beyond excellent. The music rocks, the moves fit the music beautifully and are a great mix of low impact "dancy" stuff with plyometric blasts and other more straight-up athletic moves. I never do SJP, but would buy this one again like a shot, just to have SB.
Cardio & Weights is an incredibly useful workout to have in your fitness library. It is perfect for days when you don't want to burst a blood vessel and still get a good cardio session in, or when you have taken some time off and want to ease your way back up to higher impact, more intense routines. You can do the program as is, or separate the cardio from the weights and do one after another, or just do the cardio before a separate weight training session, it lends itself to that.
Imax 2 seems to be the most popular of the Imax workouts. It certainly has the best music and higher "fun" factor, which is quite a feat considering it is a tough interval workout.
Value for money should steer you toward a C&W/Imax 2 purchase. But I think your best bet is to get one of these DVDs now and the other later when the pennies are more plentiful. What you choose first depends upon what you prefer to do for cardio training, how fit you are right now and where your passion lies!
It's a tough call. I'd get Step Blast/SJP first for sheer fun-ness in the New year!
Clare