Originally Posted by Unregistered

Just a bunch of practical reasons:
(i) Less disruptive to your CV, which is important if you want to apply elsewhere - recruiters / firms don't like it if you move firms too frequently.
(ii) Less likely to have to deal with the silliness that sometimes comes with changing firms, e.g. probation periods, bonus getting pro-rated / docked, pay getting docked, your PQE level taking a hit etc.
(iii) Easier to adept, as you already know your colleagues, the wider firm culture, the tech and admin aspects.

No harm in seeking other firms concurrently, just that all else remaining equal I would always choose an internal transfer.