The internal DH is 6:1 on the inside then when it comes out of the mast to a block on the sail it doubles it to 12:1.
I used to think that ext would be better (easier to see to make sure nothing is screwed up) but my internal works pretty well.
The internal keeps everything out of the way but it is a pain to replace anything (mast down, base off, trying to get the blocks out of the mast, rerig, get it back in without twisting it, etc).