I do both.....
TRS-
If you like to make routes as its very easy to rather than it is a pain in the butt to do it in MSTS
If you like switching its a lot easier to do in here than in MSTS
MSTS
I use MSTS for just hauling trains from one side of town to the other side of town
Cabs and sounds are a lot better in MSTS than TRS
I am sure there is a lot more that others can name...But this is just a few that I can come up with