Really not sure what the question is here but here goes:
FAS3050 clusters were shipped as 2 separate heads which together would make a cluster.
the FAS3250 can be shipped with 2 controllers in one unit I believe. There is no difference except that the 2 controllers are in the same unit which saves on space and cabling, however the downside is that there is less expandability. I believe that FAS3250 can also still be purchased as 2 separate heads, and the space where the second controller would go can host an IO expansion module.