The answer will rather depend upon how you define "preferable". MBB will look better on your CV if you wish to pursue a career in consulting, finance, or with big business adn give you access to a wider global network. On the other hand, Human Capital (or a similar boutique) may offer a better working environment, friendlier team, better projects within a particular niche, earlier client exposure, and better contacts within London.If you are already set on your career direction, a boutique with that speciality can make sense. If you are still searching for direction, a larger generalist firm can make sense.