Is it easier for an American to get a British citizenship, or a British to get an American citizenship?

My boyfriend lives in the UK. We have talked on and off about marriage, but he wants to wait to talk about it when we can talk face to face, which will be in a few days. (I finally get to see him again!) One of us will have to sacrifice in order for us to be together, I’m game either way, even though I have family here. But which one would be easier, timewise and financially. Thank you :)