A lot of people's opinions on the matter, in my limited experience, seem to be shaped based on their thoughts regarding having below natural estrogen levels. If you prescribe to the belief that the body needs a certain level of estrogen to be healthy, you'd likely opt for nolva and/or A-dex. If you don't feel there is any need for estrogen in any considerable amount, you'd opt for letro.
I don't really buy into the male body needing any estrogen and therefore have opted for Letro so far. I've been pretty pleased.