If you are in dire need of an AC, assuming that you have spent your entire life living in air conditioned rooms, go for AC room(AKA NRI Hostel or Nelson Mandela block) otherwise choose the hostels with common bathrooms.
I believe I can give you a nice advice to choose between NON AC hostel rooms with and without bathroom because I have lived in both kind of rooms. During first year of my engineering I opted for the room with attached bathroom which you might feel is better but it’s actually not. Since you’d be mostly in classes when the cleaning lady comes, so your bathroom will get cleaned only over the weekends provided that you are planning to stay in the room. If the room is locked, your room as well as the bathroom wont get cleaned even on weekends. However, the common bathrooms are cleaned twice daily. As far as the bathroom queues are concerned, even the attached bathroom will be shared by three people so if all of you wake just in time to get ready for the class, there will be a queue even in the attached bathroom.