Hi there and congrats on your offer.
I agree with Ian only partially: it is true that consulting does not teach you many technical skills and you mainly absorb a lot of theory, however:
1. If you are affiliated to a specific practice at an MBB or work in a boutique firm that is specialised in a particular industry, you learn a great deal about the field and become at attractive candidate to a start-up in that sector;
2. If you want to found a start-up yourself down the line, your consulting past helps immensely - something I can testify from my own experience (my firm is actually in medtech/digital health): you become more autonomous, versatile, courageous, organised - the list goes on.
Now on the topic of boutique vs. MBB - naturally, it depends enormously on the boutique firm, but I know that some juniors in boutiques might feel frustrated at times, because the ladder is not as well-structured and they might feel like they are alone without a case leader; the firm might lack tools (such as ppt macros) and processes (such as legal docs signing with a client), which complicated daily work, etc.
Bottom line, I personally think consulting is a great bootcamp regardless of the firm, full stop :)
Best of luck!