Hello Chandani,  

Both the Universities are quite close to each other. As everything have their own pros and cons.

Taking about in reference to placement, St. Xavier college is better than JADAVPUR University because the Placement cell of St. Xavier college is totally within Xaviers Placement cell. Which gives you the opportunity to work in abroad also. 

St. Xavier college gives you much  more than education. Like ; Extracurricular activities, mind changing activities and so on. 

Hope it helps. 

Thank you.