Having worked for the big4 as an analyst, client's as a senior consultant, and now a couple of boutique consultancies as a manager I have seen many sides of the coin. It would be as follows in this order:
Ownership and responsibility
Work Life Balance (inc travel)
Everywhere has sales targets once you are a manager or above and boutiques will typically expect more of you from an ownership and pipeline perspective. Saying that a lot of it depends on whether you can build a relationship with your clients and the firm's directors.
Before anyone says it no one joins a boutique for the salary unless it is very specialised .....