For mine I'm thinking 25mm Birch Plywood with sacrificial MDF tops. If this proves not so good I'll probably move to a T slot bed with MDF sacrificial tops.
I was planning on screwing down my work pieces to the top but I may drill the birch at regular intervals and use bench holdfasts such as these Buy Axminster Bench Clamp from Axminster, fast delivery for the UK but they may not hold the piece well enough.
For some work I plan to use a small vacuum bed that will clamp onto the main bed, I was also need some raised beds to bring thin stock up to the cutter so that the Z axis is not way down when cutting.
So my thoughts are that the main bed would be more for support than work piece fixing. One good thing about the bed is that it can be changed later with minimal disruption.