Hi there. As you say both are commonly used to treat MAV so it may not matter which one you start with, i.e. you can always switch if the result isn't good or if the side effects of one are too much. If weight is a factor then topiramate does often cause weight loss whereas sodium valproate is known to do the opposite. But each person is different. I have tried topiramate (and I find even a low dose does help) but I have never tried sodium valproate.
How come you have been given two opinions? Have you seen two neurologists or is one from an ENT consultant? Of course your other option is to go with the opinion of the doctor you trust the most, or the one you think would be most willing to switch you if you asked!