osb- flat, stable but outgasses voc's alot more than any other product.
plywood-unless you are getting the thick stuff it isn't very flat unless you get the Baltic/Russian product which has to be seen to be believed just another heads up that I got from
betsy-tom (betsy-tom)'s Photos | SmugMug
After reading about his I got some and can tell you that it is perfectly flat regardless of thickness, the number of plys are astounding when compared to the domestic plywood, but it is a very soft wood; which is why they are covering the replacement table with plexi and it is VERY dusty when being cut; you will find that the edges take to being routered as nicely as solid material IF you know how to use the machine.