I actually think Rhythmic Step is tougher. I think, because I do interval workouts so frequently, my body's become a wee bit acclimated to intervals. Because of this, steady state cardio is quite challenging for me. But I enjoy both of these workouts immensely.
If I wake up and simply do not feel like working out (I'm an a.m. exerciser), I will put in Step Blast. This one is so motivating and fun. But Rhythmic Step is fun, too. Also, Rhythmic Step gives me DOMS in my legs and glutes, which is always a good thing.
ETA: I guess because of the blasts at the end, most people would say Step Blast is more intense. I'm sure I'm in the minority when I say that I find Rhythmic Step the more challenging of the two. (I also add power to the majortiy of Rhythmic Step's moves.)