I have both a 46 and a 60 and I usually chose which side to use based o if it will fit in the room I am in or if outside, the wind. In a small space or on a windy day, I always use the 46. Although a 60" will do a better job lighting a group, if I could only have one, I would choose the 46.
I wrote a full blog post with samples outlining my experience with the miraculous modifier!
http://johngress.com/photek-softlighter-ii-umbrella/