Internal ones have their pros and cons.
I suppose on the latter, it's harder to pad out (as if we would!) experience and certain achievements. However, the major positive is you're not going into this blind. You know how things work internally, corporately etc, in short, you know the right things to say and highlight on you application.
Complete the application as you would for any other job, except rather than the touch of guess work at which aspects of your career and skills you highlight, be extremely focussed on those you know will be relevant to the job, department etc.