Because we have reused all the old R200 disk shelf's (20 in total), it was very clear that a FAS20x0 was not possible. I believe that a FAS2050 only can have 6 more disk shelf's. You must check the amount of disk shelf's (not only the max capacity) if you want to reuse the existing shelf's.
Our fist idea was a FAS3070 (load and amount of shelf's), but we even have decide to go to a FAS6040. Future grow was very important.
In general, I believe that a FAS3xx0 is a better solution for a R200 replacement, but everything depends on the situation.