I don't think it matters, but I don't know for sure. I think the MVP/Gold/75k/100k buttons are just syntactic sugar for the underlying fare class you'll be booked into, which ultimately is what drives whether you'll be instant-upgraded or not (assuming sufficient status and availability of "U" space). So I think that if Main and First Class Upgrade end up booking you into the same class, you'll be instant-upgraded regardless of which column you chose (again, assuming sufficient status and availability of "U" space). But having said that, I've never actually put it to the test.