I'm curious about that also. I've been using a 4" Parker 974 for ages and it's always been up to the job, unless it simply won't open wide enough. (Rare, but that is why I want a pattern makers vise...)
Of course, If I get the base sorted out for the Parker 386...
What I always wanted (but never had and don't have the space for) is an 8 to 10' bench with a pair of identical vises, one at each end for those times I want to clamp a long piece. Probably a better way is one on the bench and one on a stand at the same height...