You might get selected in both, IIT and AIIMS. However, you select Medical over Engineering only when youre passion is Medical Science and you prepared much more focussed for that. If it is the other way round, you focussed mainly in Engineering, parallely keeping the Medicine option open, youll probably select IIT Over Medical. Secondly after IIT, you probably have chances of getting a Strong Job Offer, however after AIIMS, unless youre interested in Government Services, you probably again need to prepare for AIIMS/NEET PG, for Postgraduate Studies in Medicine, for a Strong Medical Career.