For my welding table, I used 3/8's thick steel. Plenty thick for what I do.
However, for my workbench, I used the cheap butcher block kitchen counters from Ikea. For the price, it can't be beat. Not the best butcher block out there, but who would want to use the best you could get for a bench that's going to get worked?
IIRC it was like $70 bucks for a six foot long counter depth sheet.
Has to be oiled, the oil is only about $3.00.
I build this because I don't like to work on the steel table as a work bench. Beats up stuff and gets it dirty and oily. Stuff slides around too much on metal when you try to work on it. If you have something that needs to look nice, better to use wood work area as welding bench will trash nice stuff when it gets dragged around it.
I also have a stainless steel top on my tool box. That's nice as well as it's very easy to clean. It's a good place to put something dirty down with out having to worry about making a mess.
-Stumbaugh