In my experience, the main difference will be the level of expectations:
- Both MBB & TIer2 will have a case + fit portions. Yes, some MBB and some Tier2 will also add specific sections, but this is the general structure
- MBB cases may be a little harder on average, but this is not systematic
- The big difference however is in what we expect of you. At BCG/Bain (interviewee led), we expect you to completely drive the case, formulate hypothesis (usually verbalized), and really drive to an answer in a MECE, structure fashion. You need to make inferences, do mental math quickly and accurately... All these apply in the top tier2 but at a smaller level of expectations - and they don't really apply in the bottom of the stack
=> this is really indicative of the kind of work you will be expected to do once in the consultancy. Some Tier2 are very similar to MBB in terms of actual work (not in terms of reputation or branding... or fees); most just aren't anywhere near this.
Last thing: there were 60 candidates for 1 position in my BCG office when I applied; ratio is even worse in some specific offices. If anything, I'd say this has gotten worse since then, and the general level of prep is higher now that there's so much information on the web and companies like PrepLounge to help you prepare. I do not know what the ratio looks like at Tier2, but would expect it is often significantly lower