Longer focal lengths compress the image more and result in a blurrier background. Like take a 70-200 and take a picture at 70mm and then take the same picture at 200mm with the same settings will result in a much blurrier background.
Why do people always write that longer focal lengths compress the image, when they actually expand the background?
Compression will give a sense that objects are smushed against a flatter plane of view, like against a piece of paper. Its flattering for portraits and macro.
Expansion will make objects seem farther from each other than they really are. Ultra-wide for example make rooms seem larger than what they really are. Great for real estate and unique compositions when getting close.
I love 50mm's as they do neither, But back on the OP, Id get the 100mmL. Its got hybrid IS which is awesome.