I'm in the process of building a workbench myself. I'm using ash for the top with a hard maple skirt. My thoughts were to make it strong and durable, and I never really considered ease of flattening. Considering it now, I think it could go either way. The softer wood would be easier to flatten, but it would have to be done more often. Guess it comes down to whether you want a difficult job you do occasionally, or an easier job you have to do more frequently.
I don't want to hijack the thread, but since my dilemma is closely related and may be a question the OP encounters as well, I'll post it here.
Right now I'm working on designing the vises. I'm going traditional, with the exception of an acme threaded rod in place of the wooden screw. I'm thinking about lining the clamping surfaces with bubinga, since Woodcraft has it on sale right now for 1/2 price (7/8 x 4 x 24) and I can use the cut offs to make planes and other tools. Got a router plane and shoulder plane in mind already. (Been watching too many Paul Sellers videos.)
However, now I find myself wondering whether the vise jaws should be hard or soft wood. This is all speculation on my part, but I'm thinking a softwood may grip better and have less chance of it marring the workpiece. Hardwood, on the other hand, would be more durable, possibly stronger and more rigid, etc...
Thoughts anyone?
Working wood with wedge and edge, catching fish with feathers and fur... -Roy Underhill, with addendum by me.