Thanks for the advice guys.
I can definitely see how a windshell alone would probably be good enough in terms of warmth and blocking the wind (meaning a modern softshell is not "necessary") and on the other hand why a softshell would offer more convenience in some areas (probably warmer, better features).
I ended up emailing the guide company to ask if I could could take a wind jacket or two instead of a softshell. For better or worse they said "no". So it looks like I'll be bringing a softshell. I was keen to try out a wind jacket on a big mountain in the snow but, alas I will have to wait. Oh well, I'll live :-)
(Fortunately I already have a softshell. I think it's the Arcteryx Gamma LT hoody, which I imagine is similar to Arcteryx Gamma SL that rOg_w mentioned he uses. In case anyone's curious, I've used it on some day hikes in cold weather and hiking up Mt Whitney last year and really like it. It's uninsulated, very durable compared to most wind jackets, breathes fantastically, and has an awesome hood. The main downside in my limited experience is that it dries relatively slowly and weighs at least 6x as much as a typical wind jacket.)