The way I've understand the internal/external thing is that, in an external style, your goal is to win a fight against an opponent (you work on something outside yourself), in an internal style, the goal is the win the fight against yourself (you work on something inside yourself).
This also explains why most external arts (like karate, kung-fu, TWD...) give a greater emphasis on competition than the so-called internal ones (like tai-chi, aikido, etc...).
That said, I don't think there's one martial art that's really 100% internal or external. A 100% external MA will put no emphasis on concentration, balance, etc... A 100% internal art will be like sitting and doing meditation the whole time...
So, every martial art is a blend of the two. It's just that the proportion of each element is different, and the art is put in the category of it's main element.
To make it short, external may be viewed as "Taking control of someone to learn how to control yourself" and internal may be viewed as "Control yourself before being able to take control of someone else". So, in the end, everybody is on top on the same hill. It's just the way to get on top that's different.
My 2 cents...