Until either one is proven, many people tend not to believe in aliens or in time travel. While they both seem likely to appear in a sci-fi novel or movie, realists laugh at those who do believe in those things.
Personally, I believe that time travel has a lesser likelihood at happening in the real world than aliens contacting us or us contacting them. If time travel were possible, the travelers would only be able to go backward in time because the future they once lived in would no longer exist. In this instance, this up backwards would be called that- just going backwards in time. In order to time travel, I think that people would be able to go forward and backwards in time, not just backwards.
While I like to pretend that aliens don’t exist because the idea that we are not the only living creatures in the universe terrifies me more than anything else, there is a higher chance of being contacted or contacting aliens than tie travel. There is no way that we are the only living creatures in our universe considering how absolutely massive it is. Earth is the size of a grain of sand compared to our universe, not to mention all of the endless other universes out there in our galaxy.
Time travel would involve ripping the seams of time and stitching it back together without messing everything up forever. Aliens just involve other living things out there in the void of space wondering if they too are the only beings out there.