There is nothing wrong with a current Marshall cab if treated properly. Some people like the MDF back better than the ply so that's up to you. But they aren't up to really harsh treatment like some of the premium cabs. Funny thing is the more someone pays for a cab the easier they treat it. As far as sound they sound like they sound neither better nor worse just different.
Putting the right speakers into the ab will probably have more of an impact on your tone than the birch ply / mdf trade-off. If you like V30's (I do as well, although many here do not), I would expect you to be happy with the results regardless of the wood.