In the words of George Orwell: "Four legs good; two legs bad."
That said, I personally don't like tables with casters. When you bang on them, they move a little, and it absorbs the energy -- and has to be moved back. But I know I'm pretty much alone in that opinion.
But you don't want to be pounding on a table that's tied horizontally into hollow concrete. Concrete is great in compression, but that's about it. Steel legs are going to be better at transferring energy to the floor.
I agree with you completely. I just built a 3x5 welding table, with industrial machine leveling feet (4-inch rubber pads with 3/4-inch threaded rods to adjust it). The feet will protect the floor while beating on the table, levels it, and it's
It is HEAVY.
To the OP: If you've got the room, put the table away from the wall and leave it there. Build your shop around this table, rather than fitting the table into your shop.
Mine is 3x5, due to the piece of 1/2-inch plate I found at the scrap yard. I wouldn't make it any wider than 3 feet, and no smaller than 5 feet... six would have been better.
For the frame, I used 2x3 c-channel. The down-standing legs are 2x3 box tubing, 3/16-wall.
A 2x4 piece of 1/4-wall tubing runs along the bottom of the legs (front-to-back on the table), and the feet go into that piece.
A piece of c-channel runs side-to-side connecting those feet.
It weighs about 700 pounds... which was REALLY nice when I threw a Ford 9-inch rear on it and it didn't move, and when I had a Corvair engine on it the other day and needed to lift one end of the engine to set a block of wood under it--I climbed up onto the table with the engine to lift and move the block... the table never even knew I was there. It was like climbing up onto a concrete slab.
Anyway, look at employing C-channel for your construction: it's strong, sturdy, and easy to work with.
-Brad