Hello Chinmoy,
It's hard to compare different branches of St.xavier .
Both are very good colleges for UG Courses .
I suggest you ST Xavier's college Mumbai is good for BMS because it has introduced in 1999.
St. Xavier provides more internship and industrial tours than compared to St. Xavier's Kolkata .
Though Mumbai is a small campas it provides to have a good college life which should be remarkable in student Life .
Thank you
All the best