I only have the Red Pants, however, apart from the hair, the base sculpt/paint, etc is all the same.
I too didn't overly like the sculpt and paint of the Black Dress hair, plus when I got mine, that version was well and truely sold out, while I was able to pick up the Red Pants version, brand new at a local shop for a very good price.
In many way's, what would have been best would have been the Red Pants hair, on with the Black Dress, but can't win them all.
At the end of the day, I'm not sure you can totally go wrong either way and if you can get a good condition, Black Dress Ex or Reg cheap, then go for it. However, I'm not sure how easy it will be to even get a Red Pants version at a resonable price, where as the Black Dress Ex is likely to cost a bucket load, assuming you can even find one.
Best of luck in the search.