If you have time, it's indeed a good idea to crash test your interview at firm that you are not targetting directly. For the arguments,
- you can re-use some of the one developped for MBB (challenging environment, carreer opportunities), without reminding them they are not at MBB level...
- Some boutique firm offer higher salary to junior to appear more attractive
- In a boutique firm, you developped a deeper relationship with the teams. Since its smaller, you'll work more often together
- In-house consulting : good opportunity to combine consulting job profile (working as a team, in short engagement, variety of missions, etc.) with what one could argue as more concrete relationship with the company's activity since you're working from the inside. It then offer perspective for future development within the company / industry.
Hope this helps