I'm not a car audio pro...but IMO, I like sealed boxes. They're smaller and you get tighter bass out of em. Ported boxes I believe are supposed to be deeper...but ported enclosures also need to be bigger.
I've never used anything but MDF for any of my sub builds...but I've been told if you get into applications where you're really pounding the subs, MDF can handle the pressure put on the box whereas people have been known to blow up boxes made of plywood. I don't know how much truth there is to that...but I stick with MDF. I often use 1/2" though even though many people say 3/4" is necessary.