Hi,Soham.Opt for 11th, 12th after 10th standard.11th-12th students sre generally better in mathematics and apptitude skills whereas diploma students generally face difficulties in mathematics Also there are more seats available for 12th pass students in Engineering colleges so it becomes easier to admit in Engineering college.And for Diploma students Engineering seats are very limited so there is competition.

So doing 11th-12th would be better than diploma  if you are keen to get into an Engineering institute.