Dear Dipanwita 

If you really wish to compare the two then undoubtedly according to me, doing a DNB in a clinical subject post MBBS comes out as a clear winner when compared to the option of pursuing a PG diploma. This is when we consider the aspects of career progression, scopes of hands-on training and remuneration, which comes with pursuing the two fields. All of these aspects are better explored and better rewarding in a DNB degree compared to a diploma.

The career progression in a diploma degree is only limited given that it stops after a point because a diploma doesn't come at par with a masters degree and thus prevents one from moving up the ladder after a while. On the contrary, it has time and again been specified by different authorities including supreme court of India that a DNB degree is equivalent to MD/MS degree. 

However, choosing between the two will also depend upon where your personal disposition lies and the course one gets. 

I hope my answer helps.