see problem is this, the issue is not which is the better squad in terms of what it does, but also in terms of what else it brings, as others have pointed out, there are varrious choices that require an =][= to be present, therefore if you want to take one you take an =][=, not to make an =][= effective, unles you get specialised (an =][= with hammer of the witches is very usefull against ulthwe armies for example) the only really effective use is as a heavy weapons squad.
The retributors brign you heavy bolter death, they are more resiliant than an =][= squad and give you a point of faith plus the ability to use acts of faith with those heavy bolters (think divine guidance)
so you take you pick based on what else, personnaly my prefered possition is to go all sisters