Should I prefer IEM Kolkata IT or Heritage institute technology ECE?

Doing BTech from a WBUT college is not going to help you much with anything. I know a number of Heritage ECE graduates working in software companies. Also, very few IEM ECE graduates get a core-sector job from the campus placement.

• If you want to pursue higher studies, choose a stream you are interested in. IT and ECE both are good options. These two branches even have many topics in common such as Circuit Theory, Control systems, Microprocessors, DBMS, Data Structure, Analysis and Design of Electronics systems, Communication, Financial Management, Industrial Management. 
• If you want to go for management, the choice of the branch does not matter. In case you are applying for GATE or GRE, choosing IT or ECE will help. 

• There are more placement opportunities in IEM than at Heritage Institute Technology. But the Indian IT firms do not differentiate between branches. Sometimes, they don’t even differentiate between degrees like BTech, MCA or MSc too. For securing placement you only need the required skills. However, placements depend on the academic year on which you are passing out, and the industry requirement.
• IEM has got discipline, punctuality, and strictness when it comes to studies. The college also critically follows a strict dress-code. IEM is getting transparent and does not ask for unnecessary fees. College life at IEM is similar to school life but at the end of four years, you are likely to have a secured job. 

Both of these colleges do not offer world-class facilities and teaching. So, don't expect that. There are few good faculties in IEM. The placements are mostly from mass recruiters such as TCS, CTS, Wipro, and Infosys of around 4 LPA packages. Heritage is also a good alternative. But IEM is more reputed among students.