Advantages of Diploma are that in Junior Colleges (11th and 12th) students waste a lot of time and don't acquire any in-depth knowledge of the subject, they are only studying to score marks while doing a diploma offers you great knowledge and hold on your subject. And moreover, if you have a good percentage, you get the lateral entry option

Now, the disadvantages are, not all good colleges have the option for Lateral entry and doing only diploma doesn't give you enough credibility to bag a good job. So, if you failmto secure admission through lateral entry, you'll have to give CET and then secure admission. You would end up wasting a year if that happens.

Now coming to BE, the advantages are that it has a broad spectrum of opportunities and once you get placed, you don't have to worry much and the pay gets better with your experience.

The disadvantages being that companies prefer experienced mechanical engineers over new recruits.