Thanks to all for your posts. I still don't know what MDF and OSB really are, but based on these posts I'll probably opt for the plywood. I like David's idea of being able to screw into it if needed. And David is right--I meant a worktable to build my boat, not a wood working bench (I remember using the solid-as-a-rock maple kind he mentions during high school woodshop, oh so many years ago--wouldn't I love to have the space for something like that!). My previous boat was built on a 2' x 16' set of cheap closet doors fastened together and balanced precariously on saw horses out on my back deck. Having a building table to get the boat up to waist level is imperative in my mind, but this time I want something more stable, and I can just fit a 2' x 16' table in my small one stall garage. The car is just gonna get thrown out into the weather so I have space to do this important boat building stuff.

I'm not a serious wood worker and have virtually no tools and can't do anything fancy, but have access to a hand circular saw which I will use to cut some plywood up the middle and whack off some 2 x 4s for a frame and legs. This will be way more solid than my previous building table. I can do a lot by hand once the pieces are the size I need. Thanks again...I'm off to Home Depot.
Bob