IMO if you have a 35mm that you use for most walkaround and general purpose applications, and you have a 100mm for portraits, you're not going to see a whole lot of difference between the 85mm and your 100mm. Personally I don't see a lot of difference between those focal lengths - that is to say, I think about them the same, and one's just tighter and one's a little looser.
Whereas, my thinking and my approach for a more dramatic difference, 50mm vs 100mm or 85mm vs 135mm, for example, would change fairly dramatically. I can't internally visualize a FOV difference of 15mm at that FL (85 vs 100).