McKinsey Digital is a Former BTO. Working there has the following differences with a generalist:
1) More trainings - from my experience BTO guys were constantly travelling
2) Separate staffing - you will have a different staffing coordinator who has to approve all your projects. In general it gives you more opportunities since you can also be staffed on generalist projects.
3) Different project types - BTO is working on IT strategy, Agile transformations, etc. It also depends on the country where you apply - while in US you can expect many BTO projects, in other countires most of you projects will be in various industries and functions
4) Career opportunities - again, depending on the country, the path to the partner may be easier in BTO since you have a separate group of people supporting you and dedicated projects. It may also be difficult as well if you don't have a proper BTO organization pyramid in your office
Personally, if I had to start my path at McK from a scratch - I would go to BTO