I like a thicker top, for the reasons ccasey pointed out. But if you are having to handle this table by yourself much, the lighter the better. Your shop just can't do without at least one good sturdy table that will take some abuse.
John Wright
edit: for a top that is 3' x 6' or 18 square feet...
1/4" plate will be roughly about 183.78 lbs.
5/16" plate " " " " 229.68 lbs.
3/8" plate " " " " 275.58 lbs.
1/2" plate " " " " 367.56 lbs.
5/8" plate " " " " 459.36 lbs.