AIIMS test your knowledge qualitative and NEET tests your knowledge quantitatively, So AIIMS is really tougher than that of NEET exam, but at the end it all depends on your preparation level, and surely the level of chemistry and biology in AIIMS is higher to that of NEET and so for AIIMS prepare accordingly and also don't leave your basic knowledge because AIIMS papers are logic based which is different from NEET completely.