If the top of your bench will finish flush with the frame, a good, low-cost option is to use plywood for your workbench top. The surface will be relatively durable .
2×4 jointed, planed, and edge glued, with a removable plywood, plywood. It's the best out there to help you think through your workbench.
Since this is my first bench, I'm thinking of going OSB or Ply for the top – I'll tackle a more serious bench top when I try for something more complicated. Pretty much my. bench top from ply. 2X4 and ply panel construction.
I ended up screwing and gluing 2 3/4 sheets of plywood to the top and bought a. . I ripped a 1/4" off ea. side of 2×4's, glued & pocket screwed.