https://www.cnet.com/news/iphone-x-galaxy-s9-which-one-to-buy/

If you're not interested in payment plans and price is the deciding factor, then read no further than this paragraph. The cheapest iPhone X will cost you US$999 (£999, AU$1,579). Expect to spend less on the S9 and you get two price points to choose from. Prices vary from carrier to carrier, they run for about US$720 (£739, AU$1,199) and US$840 for the 64GB version.  

Android Oreo 8.0 vs iOS 11 is pretty much like comparing Apples to oranges (ahem!), but switching from either is a big deal. If you have deep ties to one or the other, this might be another easy deal-breaker.