If your BB ply is twisty I would find another supplier if possible. If they can botch ply storage, who knows what'll happen to something built out of their mahogany?
That being said, ply is lighter than MDF and you should be able to take any bow out with the attachment of the fences unless it is really unreasonable. If so, see the opening sentence of this post ;-)
MDF will work fine but take care on your design. If the "bed" is of any size you can have issues with it deviating from flat (is that the PC way to say "lose it"?). Unsupported MDF can get real creative.
If you are like a lot of folks, you will make a few sleds before you are truly happy. If this is a first run, make it out of whatever is handy. Take what you learn from this and apply it to your "keeper".
Some things that keep a sled from becoming 'sub-optimal' rather quickly are:
- an adjustable rear fence
- a replaceable insert for the blade path
- a replaceable insert for the entry point of the blade into the rear fence
- a dummy block to cover the blade as it exits the rear fence
Any sufficiently advanced technology is indistinguishable from magic.
- Arthur C. Clarke