This applies to the Delta interview mostly, but Fedex has the same cog test.
When I went through a couple years ago they were separate. EC was more for the HR portion with some technical/knowledge help. RST was more for the cog/job knowledge portion. I'd recommend both based on my experience!
I think if you buy the EC package you get a discount to RST. Both were good prep, the EC in-person seminar was really good. And RST was definitely great preparation for the cog and job knowledge test. The RST facebook group was also a very valuable resource as they post "ride reports" (basically just up to date interview gouge) on there all the time. Good luck!