TenHammers you must be reading my mind because I hadn't chimed in on table mass yet. That was Dodge.
I weld anywhere I need to. These table threads overlap at my place
Get the thickest you can afford and biggest you can for the size of your shop. Keep an eye out for something better, it will come along.
My smallest welding "table" is an 8" square piece of 3/8" cold rolled with a bar welded under it to clamp in my 8" vise. Have built many, many things on that "table".
My biggest table is 4 1/2' X 6' X 3/4". It lives outside in front of the shop. Have to move it in the winter so I can plow snow....did this yesterday, 4" of snow today! This is the table I will weld things
to when necessary. Quickie built jigs etc. It is open edged all the way around for clamping to. I am careful not to beat on it too hard around the edges. 3/4" plate will bend. 1/2" plate actually bends fairly easy on edges or corners. 3/8" is plenty good for a start and 1/4" is way better than nothing. Like I said, be on the lookout for your next table or the next top for yours. I am looking for a 1" or 1 1/4" top for this one.
Inside the shop there is a 5/8" topped table with leg vise next to the forge. There is another two wheeled/one legged table with an 8" vise that has a 2' X 2' X 2" top. Table weighs over 600# plus whatever junk is on the shelf. I don't weld things
to that one as it is dead flat and it's staying that way. That is my "precision" table. I also have a cast iron table that is 28" X 48" X 28" (built the above vise table to match the height on this one)that is smack dab in the middle of the shop and is where most of the welding gets done. The top on that one is 2 1/8" thick and it weighs over 2200#. It doesn't move when you lean on it
Edited to add this: I have built a good many things using a piece of 8" X 8" I-beam as a "table". If you find a piece that has no twist in it latch onto it. With enough clamps you can build about anything on a straight piece of I-beam. Built a whole trailer on one one time.