I buy the special edition if it's out, if not the original.
If i buy the original then buy the special edition, i will only sell the original if it has nothing extra than the SE. This isnt always the case, for example, The Bourne Identity original has DTS and audio commentaries, while the special edition doesn't; it has extra documentaries etc.
Of course, this could just be a shameful way of making buy two versions, so if i come to a film that already has an original and SE available, I will go with the one which has the better extras in my opinion. For example, again with The Bourne Identity, I'd buy the original because a DTS audio track and audio commentaries are far more interesting and useful to me than a few extra documentaries.