Compare the summer and final placement reports of both batches for all the 3 years. Look up the student clubs and activities on both campuses. They are absolutely necessary for your 2 years there. Finally, look up the professors in both colleges, majorly for the subject, you will probably be specializing in. Good professors impart their experience along with education. Keep in mind that IIM Bodhgaya, however, lags behind in one vital factor i.e., the number of students. In all probability, the students in the upcoming batch (your would-be batch) in Sirmaur will be more than if you were in Bodhgaya. A healthy amount of interaction matters a lot, especially because it's a business school and you benefit from the range of ideas numbers can provide.
However, the final decision is up to you entirely. Assign weights to the above factors and decide accordingly. After taking the decision, do not second guess your choice after going into one of the colleges. Make the best of your time there. Chase excellence in every facet of your MBA while you are there. If you do this, the choice part will cease to matter in a few months.
Both of these IIM’s are newly built but IIM Sirmaur is more preferred by students due to the following reasons.
IIM Bodhgaya lags behind IIM Sirmaur in the number of students. You will have more peers at IIM Sirmaur which offers better scope for peer learning.