The admission of IMS, BHU is through NEET Entrance Exam & to get admission in AIIMS Patna you need to appear AIIMS Entrance Exam. Both are the decent college but in the comparision of AIIMS Patna IMS BHU is the good option to Opt as it is central University. You can also check the genuine reviews of the colleges with the given below link.

https://medicine.careers360.com/colleges/institute-of-medical-sciences-banaras-hindu-university-varanasi

https://medicine.careers360.com/colleges/all-india-institute-of-medical-sciences-patna

Good Luck!