While some people have used the Transfirmer for other, non-Firm, workouts, I wouldn't recommend it, especially for workouts like Cathe's where there is more impact. The plastic is harder and has less 'give' than that of the original club step, and could lead to joint and/or overuse problems, IMO.
And though I haven't seen any warning on the Transfirmer itself saying not to jump on it (like there is on the Fanny Lifer, which seems to be made of the same plastic), it definitely wasn't made for impact moves (and I'm too lazy to go searching my Transfirmer to see if there is a warning label).
The Step Co. step is also nicer to use as a bench, because it is longer. The length also adds to the intensity of the moves. And you can vary the height of the step anywhere from 4" to 12" or more, depending on how many risers you use (higher for step-ups and as a bench, NOT to step on).