It depends on what you’re welding, and how abusive you intend to be.
We’ve built aluminum welding tables for people who needed a super lightweight, transportable surface. We build 304 stainless welding tables for people who are welding stainless or aluminum on the surface. And we use A572-50 steel for our production line of welding tables. One could argue that it’s overkill because it’s the hardest/strongest material being used in the industry. A36 is a fine material at a lower price point. Not as durable, but it’s steel.
We have an entire article that compares A36 to A572-50 in case you’re interested in the difference.