Download them both and make a prototype in each and see what you like to use. As @franktech mentioned there are tons of threads on here, Unity forums, reddit, and the internet in general that list technical differences. One reason that I switched from Unity (very recently though) to UE4 is the licensing that Unity has. They have a bit of a record of having a pretty shady EULA and then making changes without telling people about it or backtracking on what they say. They also count things like Kickstarters and cash prizes (might have changed on the prizes) as part of your $100K gross income while Epic does not meaning that you may be forced to subscribe to a tier that you cannot financially support in the long run.
Their subscription model is also archaic in the sense that at no point in time prior to the specified end date will they allow you to cancel or downgrade your subscription. There have been a few threads on their forums where people mistakenly subscribed twice (dunno if it was a system glitch or what) or fell on hard times and Unity basically said “sucks to be you”. On the note of forums, there are a few very specific well-known users (by well-known I mean popular or very active) who are complete a**hats when it comes to certain topics especially if it’s about a deficiency in Unity.
Personally I’ll use Unity for mobile/2d projects and UE4 for everything else but it’s about figuring out an engine that meets your needs not just because it’s cool or uses certain technology.