Either would work. However, "You saved my life", would be more appropriate in this situation. Sometimes, people use, "You saved my neck", when they are delivered from a life-threatening situation by another person, but this phrase is also commonly used when a person is delivered from a serious, non-life-threatening situation. "You saved my life", is very specific and expresses your feelings more clearly and succinctly in this case.

Both are correct! The main difference between the two is "you saved my neck' is an idiomatic expression. Although it is possible for it to be used literally, I've never seen or heard it used in such a way. "You saved my life" can be used either non-literally or literally. "You saved my neck" means you prevented something bad from happening to me. "You saved my life" can either mean that you prevented my death or you prevented something bad from happening to me. 1. I was in the deep end of the swimming pool, but I couldn't swim. If you had not swam to me, I would have drowned. You really saved my [neck or life]. 2. I was very overwhelmed by the project my boss had me working on. There was no way I could have completed it alone before the deadline, but you helped me finish it on time. You really saved my [neck or life].

"You saved my neck" is an expression which means the same thing as "you saved my life" You can use them interchangeably. Hope this helps :)