Birch is lighter and you don't get MDF bugers. I will be using birch when I build my 4th order.
Side note, it's your car so you can just ignore my comment.....but I fail to see why you want the tweeters near your sub. You won't hear them at all inside your car. Unless your tailgating, which then I would put a component set in the trunk...still not on the box, it'll end up being a waste. Move those same tweeters to your A or B pillars and you'll be happier. But it's your build and car so do what floats your boat.