Chris, you are of course right : it depends on distances and what you want on that picture. I would use a 50mm lens to get the more environmental portrait and a 135mm for a tight portrait. This is why for this task I actually prefer crop cameras : there are a lot of fast 85mm lenses out there, by far more than 135mm, and enough 35mm to get me the equivalents ( of course in the wide angle department it's the other way around ). I also noticed that I can shoot portraits with good subject isolation with APS-C cameras - just have to make sure the background is a little further away
