While i prefer X-Men and overall think it has better movies, i think Fast and Furious offers something at the moment that not many other films can offer. I also think that many of the people in the 14-24 years old demographic are more excited in fast cars and over the top "testosterone filled" action than yet another superhero franchise that takes more time with the plot than the action.
Fast & Furious just seems like something easier to get into, i feel like Fox's plans of expansion may be a little too soon, i mean, Days of Future Past was their first X-Men film that finaly got back to the top 10 highest grossing films of the year, and right after that success they're planning to branch out into multiple spin-offs, with 2 of them premiering the same year as X-Men: Apocalypse.
When it comes to box office grossings, i fear the X-Men franchise is not a safe heavy hitter yet.