一遍 implies finality, whereas 一次 does not. So, for example, 我去过中国一次 (I have been to China once) and not 我去过中国一遍, because that would imply having visited every possible place within China. On the other hand, 这部电影我看过两遍 (I have seen this movie twice), because I saw it start-to-finish twice. 两次 would imply that I only saw bits and pieces of it on two occasions. So either works in your example. It's just "say that again" vs "say all of that again."