What's exactly would you like to know?
I have experience in working with both 2nd and 1st tier consultancies. According to my experience:
- Quality is much Higher at MBB. To give you a sense, what you are doing as an analyst at big4, at mck will be done by the research department or even outsourced students. Though it's worth to say that if your director at big4 will be ex MBB, quality will be much higher
- Knowledge base - McK knowledge base is extremely deep and helpful. You have tons of materials on various topics
- Support - at MBB you will have lots of people taking care of extra staff - doing research, the visual design of slides, sophisticated models, outsourcing for making surveys in the field. At big4 you'll do that stuff yourself
- Expenses - you will have better hotels, better dinners, better airline tickets, etc. This is important if you spend 90% of your life out of home
- Exposure to senior management may be higher, though it depends on the country. Since the projects are more expensive at MBB, you may have more senior people involved
- Relocation to different countries - options highly depend on the org structure of MBB/Big 4 company. I've seen good and bad example in both. But since you have higher salaries and per diem, at MBB you'll definitely have a better lifestyle at MBB if you do relocation