For a workbench any poly will do. I'd ask friends and family to see if they had a partial can left over and use that up.
In general, I prefer oil based poly. It seems to be more flexible, whereas the water-based seems a little brittle. It may be my imagination as both are surface finishes, but oil based appears to penetrate a bit more when applied.
Water based does seem to cure/dry faster and sanding between layers doesn't seem to gum up the sandpaper as much.