Bay,
Just some hopefully helpful info.
I know you will probably delay putting the plywood on the table, but just a few things to keep in mind:
In the construction world, half-inch plywood on 16 inch centers is the absolute minimum (not a maximum) for a flat surface so it won't sag. Homesot is glued chopped-up paper, and it will sag, and even a small sag between centers will ruin your layout. Homesot is not a structure sheathing. It is for soundproofing, insulating, and other functions.
Just in case you may change your mind about putting down the plywood before you move, remember that a sheet of half inch plywood is not that heavy. Depending on the grade you use, I think it runs about 35 to 40 pounds or so. If you are hiring movers, the table will still be very light for them to move and carry. Even a 150 pound table is child's play for movers.
When you decide to buy your plywood, I would recommend that you use get B-C grade (a B face, and a C back). It is almost totally smooth on the B side (knot-holes filled) , and this will prevent problems in the future. You can get it at Lowe's. It is really strong.
Although others may disagree, I strongly recommend against CD-X plywood, which is really the cheapest structure plywood you can buy, and has lots of "voids" on its side and interior, because of the unfilled knot holes. They sell this at Lowe's too, and you can take a look at both grades within a few feet of other. Be sure to flip them over and look at both sides.
You could probably get away with using a good grade of 3/8ths inch plywood (generally Home Depot sells it, but not Lowe's) IF before putting it down you put a good brand of adhesive epoxy down on the tops of the cross members, headers, footers and side boards of your table frame. Both Lowe's and Home Depot sell "PL-3" construction epoxy in a caulk size tube, for around $8.00. If you use this, be sure to have someone help you lift and place the plywood down on the frame almost exactly perfectly onto the wet epoxy, because once it is down, it will be "gripped" and it may be almost impossible to slide it, unless you do so immediately and use some force (like hammer taps). After 30 seconds or so, forget it. It ain't gonna move. Then, get out of the basement for 8 hours or so, because the adhesive odor will absolutely make you pass out in a closed room.
The 3/8ths and epoxy will create a very very rigid table top, but if you ever have to put alot of weight on it between the cross members, it will probably flex somewhat. This may or may not give you cracks etc. in any landscaping you put down.
Mannyrock