Join Date: Sep 2007
Location: Santa Cruz, CA
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My feeling on these type of questions is that the labor of the job (your time)is exponentially more expensive than the materials and often you are short changing yourself by choosing a less expensive material over the one you know is better. Just my two cents but I have found that after spending 80 or so hours (WAG) on a job like this it's easy to regret saving $50-75 or so bucks for cheaper materials.
Good marine plywood cuts much cleaner, the plys are usually finer grain, is vastly stiffer as Stillraing points out, has no voids, and will provide the right answer for the next potential owners survey.
Whichever way you go, good luck with the project