Hi Danyel,
Neither of those sentences are actually correct. The correct way to write/say that particular sentence would be one of the following:
- What a boring movie I saw. (instead of "What a boring movie I have seen!" )
- I watched a very boring movie.
- I watched a movie that was very boring
- That movie I watched was very boring.
Essentially, (when written correctly), they are all the same, as they are all ways to talk about something that happened in the past - as in, it "was" a movie that you watched, and is now completed, and you are now speaking about that past thing.
Hope this helps.