If you are the kind of person for whom gear is more important than results then go for the 135mm f/2 L however if you're a photographer who works in the real world, the SF is a great feature. Peoples skins aren't perfect especially the older they get, and while it works on guys the girls don't like even the smallest crows feet. So many people have body issues that one pretty girl I know cannot even have her photo taken by anyone, so phobic is she about the way she looks, which is a shame because she's quite a looker. On with the SF turned up to 2 and it returns an image she's happy with straight from the camera, - no messing about in Photoshop because she'd never manage to even view the image if it was from the 135mm f/2 and brutally sharp.
The SF is just another tool to have in the bag, it makes people happy, and because the gear heads all want the f/2 they miss the money shots. Not many people currently use soft focus, so it's something of a Wow factor when used sparingly.
If I want sharp I'll use the 70 - 200mm f/2.8 IS L MkII
Only you can make the decision as to the type of photography you want to take, but 135mm is bang in the portrait focal length