Well, if you are going to use the table to make up the bed, there is something to be said for the table thickness matching up to the bench tops at the lip where it rests, which is 3/4" on my Boler
On the other hand, you probably would not notice a 1/4" dip.
On the other other hand, 1/2" ply by itself is going to sag, so you would need supports anyway.
I think if I were making a new table, I would make something cored, with lightweight ply on both sides and internal stock. A full 3/4" table is a lot to horse around (although, with the scamp
lifting hardware maybe not a problem, and 3/4" ply is certainly simpler).
Also, if you only use it as a bed, I find a half dozen or so 1" x 8" x width-of-well to be much easier to handle, and they allow ventilation too (I have the table in storage).