First of all, congratulations! Are you aligned to the digital practice in BCG, or did you receive the offer as a generalist? The answer to your question will be highly dependent on your appetite for digital projects.
As far as I know, both companies provide similar opportunities regarding career perspectives, salary, and alumni, thus this may not be a differentiating factor in your decision.
I would also encourage you to think about the people, interviewers you met at BCG and McKinsey, and assess who you would like to work with more.