AIIMS is definitely the better option. There are many reasons why it is the superior choice. One of them is the competition. You will be competing with lakhs of students to get selected. This entrance examination is trickier than all other entrances. Irrespective of how well-prepared you are, you are likely to get stuck between choices in the examination. 

PGIMER has a standard examination pattern. They usually aim to test how thorough you are. Also, the competition is much less. Since around 15000 or fewer people appear in the examinations.