I used 3/4" Birch Plywood.
That said, I did a little more research on the subject of Birch plywood vs. MDF to try to get an answer, and I still don't really know which is better. Many of the forum threads (not on FF.com) that I found come to the conclusion that Birch plywood is better than MDF in almost every way. Then I find threads that say the MDF is way better than birch plywood and that any plywood should be avoided.
I already have mine built so the subject of which is better doesn't really effect me anymore, but if someone asks which is better, I still have no idea.
The real reason I used the wood I did was because it was what I had and it didn't cost me a thing. If I had to go out and buy the wood, I probably would have gone with MDF or solid birch board. Furthermore, for me, this whole subject of what is better is like going to a restaurant and only judging it on the forks. There are a lot more important thing to me than the subwoofer and the system I have blows away the stock (non-subwoofer) system I used to have. It may not be solid silver but it sure beats plastic forks.
Now that that's out of the way, it's time to was my car again. I cleaned it yesterday but now it is completely covered in mud splotches and road crap. I hate melting snow.