Well, MDF is thicker, firmer, and has more mass so I like it for mounting speakers. With some added RTV silicone, //content.invisioncic.com/y282845/emoticons/yumyum.gif.0556df42231b304b9c995aefd13928a8.gif
MDF will be a fair bit more expensive than plastic, but there are plenty of people here who have MDF scraps, and I'm sure you'll be able to find someone to give you a fair price on a couple 6x9 to 6.5 adapters.