I’m not sure if the reputation has decreased but to be honest you should not be choosing your MA based on that. You need to choose it based on what you are going to get out of it and what you are going to learn. Having the name of a good school behind you will honestly only get you so far…
I graduated last year from CSM and one of my peers who was outstanding didn’t get offered a place at the RCA so maybe you are actually better than you think!
However the new Kings Cross campus has given CSM such a breath of fresh air and screams better teaching standards. Also the course director who was my old director on BA Product is an incredibly smart fellow. I suggest speaking to current students of both courses to find your answers as a lot of members here seem to be from the States.