When I built my workbench about 20 years ago I used 2 pieced of 3/4" plywood for an inch and one half thick top and coated it with 1/4" tempered (on both sides) Masonite. The tempering on both sides helps keep liquids from penetrating. After years go by the Masonite gets beat up, cut, holes, etc and I did change it a few years ago but I am now thinking of ripping off the new Masonite (definitely NOT as good as the original piece from 20 years ago) and putting on a product called Sintra. The Sintra is also 1/4 inch thick, black in color and is made of flexible plastic which is liquid proof, scratch resistant and durable.
I do NOT like sheet metal tops mainly because they conduct electricity and I often use my workbench to do electrical and electronics work and might cause a short while working on them. The Sintra looks like a great product, will allow cuts and holes to be made for Vises, equipment, loading presses, etc. and would also be great for gun cleaning, repairing and maintenance.
I would DEFINITELY recommend using some sort of top whatever you do this way when that top becomes ratty, too soft, stained, beat up etc, you can simply rip it off and freshen it up with a new covering.
Just for the record........ EVERY TIME I clean guns I spread a newspaper over the workbench top so it absorbs any stray liquids, catches lead particles and debris, and gives all firearms a nice and cushy surface to be place on. I often will remove the top page of newspaper to expose a fresh and clean one during different stages of cleaning. After cleaning I just roll up the newspaper and the mess, debris, dirt, oily patches, etc. is in a nice and neat package in the garbage. On top of the newspaper where I lay the actual gun is a "Bar Mat" (a Jack Daniels one at that) with all the small rubber bumps which makes an IDEAL gun surface and will let liquids, debris, etc fall between and NOT scratch your prized firearm. I you are friendly with a Restaurant or Bar owner I'm sure then can give you or get you one on the cuff - they are free from the Liquor Distributors to them and they make the PERFECT cleaning surface for firearms as they will also keep small screws and springs from rolling off and getting lost. No matter which bench top you wind up using - get one of these Bar Matts - they are terrific!