Both PGDIE and PGDIM are very good courses for a mechanical engineer, but the syllabus for both these courses is different. 

PGDIE

1. It is a flagship course of NITIE which is more focused on industrial engineering. To get admission to this course, you have to clear the GATE exam.
2. As a fresher, you can make up with most of the students in your branch but you might not be able to contribute as much as the students who have significant experience, especially in class discussions and group activities. 
3. But there won't be many problems during placements as there are many companies who are looking for fresh candidates to join their team, especially in the FMCG sector. 

PGDIM

1. It is a management course that is equivalent to an MBA course, and to get admission in this course, you have to clear the CAT exam.
2. As a fresher, if you take admission in this course, you would probably be in the minority as most of the students would have some kind of work experience. 
3. There will be certain projects which would need some kind of experience and here your other batch mates can be a step ahead of you. 
4. You will have to do a lot of reading and communicating with all your batch mates, professors to learn how a business works. 

Both the courses are very different from each other and now it depends on you in which field you are interested in. If you want to pursue a management course then go for PGDIM or if you want to be in the field of engineering, then you can choose PGDIE. 

According to my friends studying at NITIE Mumbai, it is a decent college that provides its students with tremendous opportunities in placement. For all the courses (3 courses offered) that are offered, the students have nearly 100% placements. The packages were at par with the IITs and a handful of IIMs as well.

The GATE score cutoff for the general category is expected to be around 700-750. Since the admission process is not transparent, the marks on the basis of which the candidates are given admission is not disclosed. The expected score is rather an assumption made out of experiences of those candidates who secured scores around 700-750.

My brother used to be a student of NITIE Mumbai, so I think I can give you some information regarding their admission process.

• First, you need to crack the GATE cut-offs for admission. As per the past year, the required GATE cutoff ranges between 650-700. However, the cut-offs are uncertain as it depends upon the number of applicants.
• Once shortlisted, you will have to appear for the GD-PI process. The Personal Interview is on a profile basis. You have to do well in PI, in order to get selected. 
• After the above round, a final merit list is prepared based on composite scores in GATE, GD-PI, relevant work experience, and academics. The academics also have a significant weightage which usually remains undisclosed.

Hope this gives you the overall idea about admission at NITIE.

If you are asking from a technical point of view, you will not face any issues for doing PGDIM from NITIE once you have completed your M.Tech.

But, in the future, you might be asked during an interview as to why you have pursued a management degree after completing your master’s in a core field. Be prepared to give a convincing answer. 

If you are thinking of joining the industry and then doing a management degree makes sense. It would definitely help you scale up your current job prospects. But, if you are pursuing this degree without any industry experience, it might come off as a little haphazard, like you are jumping from one degree to another. This might confuse your recruiter a little if your final goal is to bag placement in a decent company. 

You can go ahead but think of a good reason as to why you want to pursue this to ensure that you get placed in your future and don’t face any problems.

You have not specified your category and gender and confirming that bit may be tough. Following are some certain things you should need to know.

• These parameters are as important as the CAT score for Indian B-schools, if not more. 
• For the worst-case scenario, I will assume you are a GEM (General Engineer Male), and answer according to the historical trends.
• For PGDIM, by a decent score, you should mean 98.5 percentile with 92+ percentile in every section. 
• But mind you, you will have a hard time getting shortlisted for good companies during summers and final placements owing to your academics and it's the comparison with the rest of the batch. 
• The closing percentile for round 1 in the general category is 90 percentile.
• For PGDISEM, by a decent score, you should mean 95.5+ percentile with 90+ percentile in every section. 
• Because of lesser competition in PGDISEM, you can have a fair chance of getting shortlisted in good companies for summers and final placements. 
• The closing percentile for round 1 in the general category is 90 percentile.

Apart from the decent scores, you will need to perform exceedingly well in GD and PI to secure admission at NITIE.

NITIE is not only at the same standards as the IITs but also better in few areas.

1. Legacy: NITIE has a legacy of more than 50 yearsof creating leaders for the industry whether it is manufacturing or IT Service.
2. Brand: NITIE has created a brand name for itself in these years and the industry is well aware of it. For every domain of Business, reputed companies visit the campus for the placement drive.
3. Location: NITIE is located in the business capital of the country. Due to its location in Mumbai, it gives the option to explore and learn a plethora of things.
4. Academics: NITIE is more of a B-school as compared to IITs, which are basically more known for their research work. NITIE has numerous centers such as the Centre for Technology and Innovation Management, the Centre for Supply Chain Management, the Centre for Environment Studies, etc. The students often do their projects running under these centers and experience insights into real-life techno-managerial issues.
5. Course curriculum: The course curriculum of NITIE is industry-oriented.

The current placement statistics for the institute are as follows.

Overall, NITIE is a great place to study. 

Once you get a valid score in CAT and you get a call from NITIE you can start preparing for the GD, PI and WAT process. If you are pretty confident about your score and which college you want to get admitted you can start prior to the result.

Now coming to your question about how to prepare for interview for PGDISEM:

• First thing you need to know is your profile. This would encompass individual data, academic background, work experience and your hobbies.
• Try to know laws and agencies working for sustainable earth. You should be able to convince the interview panelist.
• Particular Terms and jargons like: Kyoto protocol, carbon trading, Greenhouse Gas, The Natural Step etc. should be properly accounted for.
• Accept if you don't know a few questions. Interview panel may have professors who are consultants to leading MNCs and proponents in their field.

If you have any doubt, connect with your mentor assigned to assist you. I hope this answer will help you and any future aspirants of NITIE. Just give your best and be confident. Wear that smile of confidence and just hold your thoughts. You will definitely get admitted to NITIE. Good luck.