If you were the teacher...
I think the second one should get full credit though the first one shouldn't get any credit. It is the case that your method works under constant acceleration and the problem concerned constant acceleration. If he wanted to mark you wrong for not understanding average velocity, he should have done so on another problem where misunderstanding would have led to a wrong answer. As it is, he has no case that you didn't completely understand everything and neglect to mention the fact of constant acceleration simply because you took it as implicit.