My brother studied in IEM Kolkata and firstly, it is a reputed college in West Bengal, even more than Jadavpur and IIEST Shibpur. The placement rate at IEM is about 80%-90%. On the other hand, UEM is a regular engineering college with only a few defining features. Its fame is because of the fact that it was established by the IEM trust.

• However, if you compare UEM with IEM then, IEM wins on the faculty quality and placement scenario. At IEM, the teachers are friendly and enthusiastic.
• IEM also has a limited number of seats that are filled based on the rank in WBJEE. It does not have a management quota.
• UEM was established recently, so it is a relatively new private university and is the third of the three engineering institutions founded by the IEM-UEM Group.
• UEM is a much more disciplined institution, quiet more than IEM.

If you're keen on choosing between these two, I'd suggest you consider the student intake because if the number is lesser then it's better. For this, you can ask alumni or someone currently studying there. Some students also have a negative view of IEM. The IT department is not as good because my brother's friend is there and claims that the professors do not know even the basics of the program. 

So, be very refined in your research before choosing.