Personally, I would go with Atlantis. I actually haven’t been playing for a year yet and have never been in game during the summer event. I have seen the heroes, and, while they are good, I just think you have more options with Atlantis. There are a lot of good 4* you can pull(Wilbur, Proteus). Even the Atlantis 3* are really good.
That said, you are way too early in the game to worry about leveling 5* heroes. If you happen to get one, great! Save it, of course. However, I would focus on a big roster of 3 and 4* heroes. That will give you a lot more options. I doubt you have anywhere near the ascension materials needed to max a 5* anyway, after only 1 month of play.