Best first internship for indecisive students: Boutique M&A, Big 4 Transaction Services or something else?

Dear all,
I am currently interviewing with several boutique firms in Germany and was wondering if it is worth doing a 5-6 month internship at a *really * small firm (M&A, so I might want to do Asset Management or Private Banking as a career. What's the best way to position oneself? Just do M&A even with a no-name brand, or go with a well known brand in a less prestigious role (eg. Big 4 Transaction services or a large bank in a mid-office role) instead. Or aim for a slightly larger bank that still gives internship offers to people with no experience?

Thank you for any answers!