stiffer = better IMO.... those are the one pieces I'd prefer to have harder than necessary.... softer control arm bushings would be far far preferable... IMO.
I used 60 in my bushings since the original rubber is about 60 and I didn't need/want all the transmitted noise plus the added strain on the remaining components. They still feel good.
I imagine that if you start with fresh subframe mounts, 60 durometer will be fine. The 94 filled ones on my old 535is were still in perfect condition when I sold the car so I'll probably step down to 80 or 60 on my 528e.
