Hi Katie,
RS is definitely the closest thing to Step Blast, and to be honest, the choreography isn't any more difficult. They both rate up there as her most complex, IMO.
The only major difference between RS and SB is that when she puts it all together at the end (the Challenge), she doesn't put blasts in between, like she does in SB. However, if you buy the RS+ dvd (whenever it comes off of backorder!!), you will get two other superb cardio workouts that mish mosh together very well for some quite intense workouts.
Hope this helps,
Sandra