FIFA 18 is a good game of football with an absolutely incredible singleplayer mode. PES 2018 is a fantastic game of football wearing clearance rack kit. That's about the sum of it.
FIFA and PES are like Batman and The Joker, their entire existence defined by the presence of the other. You can't play PES without talking about FIFA's licenses, and you can't play FIFA without talking about PES' gameplay, because those things are as much a result of targeting the competitor's weaknesses as their own inherent strengths.