Here on Wattpad, when you choose a genre for your story, it can only be one and there are only limited options. For example, 'Poetry' is a really broad category and there should probably be sub-categories like 'Comedy Poetry' or 'Short Poems'. It's the same with the genre 'Teen Fiction', which is so broad that a story could be a 'Vampire' or 'Werewolf' genre and also fit into 'Teen Fiction'.
I always used to place my stories into broader categories like 'Teen Fiction' rather than 'Adventure' or 'Thriller' because I assumed that for a broader category, more people would find or be interested in my work. But, now I think I'd recommend going for the most specific genre you can. It means that people can instantly identify your story and know somewhat about what it's about. And, as I said in the last chapter 'How to Reach the Rankings', in a more specific category there's likely to be less stories and so your story will probably be more likely to get into the rankings.
If you're trying to choose between two, or maybe even more categories, it can be especially difficult, particularly if you've only just started writing it. But don't stress too much, whatever genre you initially choose, you can always change it later. For example, if you intend your story to be 'Horror' but are only a few chapters in and it seems to fit a 'Paranormal' or 'Mystery/Thriller' so far, which do you choose? Obviously it depends totally on the story, but there's no harm switching genres even daily, just to see what seems best.
I'd also take a look at the categories you're trying to choose between. For example, I had published a story a while ago and was debating between 'Mystery/Thriller' or 'Adventure'. Obviously 'Mystery/Thriller' just sounds better but when I took a look at other stories in the category they were all crime/investigation/abducting and kidnappings, so I switched mine to 'Adventure'.