When you consider that the frame is supporting as much as 500 pounds, the strength of the frame is crucial.
Solid wood is without a doubt the strongest and most desirable frame.
Screw holding capacity is also very important because the frame either screws or bolts together and the slate screws down to the frame. If the frame does not have good screw holding power, it will not maintain its rigidity. Solid wood has better screw holding capacity than either plywood or particle board. The design of the frame is also extremely important. If the table utilizes frame supports that go both the length and width of the table, make sure the frame supports the slate on all surfaces of the frame.