how the heck did you build your own head.
pretty sure mesa uses birch but Im not 100% sure it would make a huge tonal difference. I just made a combo out of pre-finished cherry ply (3/4) and it sounds sweet. but cherry and birch are heavy. I would just get whichever is cheaper(or had lying around). or you could get crazy and make the sides out of birch and the top and bottom pine.
No no no no no no! Do NOT use "whatever is cheaper" for speaker cabs! For an amp head only cab, it doesn't really matter, but cheap plywood is usually filled with voids, which tend to resonate in the most unpredictable ways when the speakers start to vibrate. Trust me, speaker cabs are expensive & heavy for a reason!