Hello.
Work perspective of the organisation is different, Both ISRO (INDIAN SPACE RESEARCH ORGANISATION)  and DRDO (DEFENCE RESEARCH AND DEVELOPMENT ORGANISATION)  are government organisations, ISRO basically incline to the area of space research while DRDO is incline to the area of military research, work of both is entirely different and they both are necessary for the proper growth of the country so it will be difficult to compare these both because we can only compare two organisations when their work area is different but here it doesn't look like this both are comparable on the same level, there is nothing like DRSO I think you want to ask about DRDO that can you do aerospace engineering or not, so I would like to inform you that it basically depends  upon the vacancy, it might be possible that in some year vacancies are zero for aerospace engineering and in other year it may be in a good amount but you can do if vacancy available then your selection will be done on the basis of your GATE score.
Best of luck.