If you go with wood, the key is not how fat the legs are but how well they are braced. My bench top is 2X4's laminated together using glue and screws vertically so the base bench top is 3-5/8's " thick, on top of that is a standard Formica kitchen counter-top with a back splash, so its pretty stout. I have 6 2X4 legs but there is a plywood back and sides creating a three sided box.
The base is simply not going to allow any significant wobble at all. I do have it lag bolted to the wall studs, but that is so I can do things like put a tubing bender in my Parker vise and go for it. It used to be deck screwed to the wall, and I did rip that loose, so I did it right with lag bolts...
There are 1X's going across in a shelf, also well braced, and there are 4 heavy tool boxes siting on that. No visible flex there either. The only thing light weight on the bench is I used 1X4's to face frame it, and added doors to keep **** out of the storage area. It adds a little more shear strength to the structure, but I can't honestly say it was needed.