The biggest question you'll have to ask yourself is...
Am I'm willing to start all over with my app library with the iTunes store?
Since the iPhone runs exclusively with Apple's iTunes store, you won't be able to retain ANY APPS from the Google Play store, ESPECIALLY any apps you purchased. Additionally, you may not be able to utilize any premium DLC or in-game currency for the free games (as those are tied to the storefront they're purchased from)... which causes some "transitional pain" from Android to iOS (& vice versa).
Other caveats with iPhones are...
* NO EXPANDABLE STORAGE -- Whatever storage space for the model you purchased is ALL YOU GET. Samsung phones support MicroSD cards (the S9 supports MicroSDXC cards up to 400 GB's), if you need more space.
* Free Cloud space is more limited -- Apple only gives you 5 GB's for free for EVERYTHING. Google gives you 15 GB's of space for everything (e-mail, documents, personal files)... but they do offer some exceptions (like allowing unlimited picture storage, as long as you don't mind Google compressing the image files. Music storage is limited by the number of songs & doesn't count storage purchases).
* You will need a computer with iTunes to perform Emergency Factory Resets -- In the event that you forget your lockscreen PIN / Password (or whatever you prefer to use), Apple REQUIRES you to place your phone in Recovery Mode & connect it to a computer that's online with iTunes to reset the phone. It's for this fact that I cannot assist customers who forget their lockscreen PIN / Password on iPhones (as I don't have an available computer with iTunes at work).
Android phones don't require a computer to do this (as it's an option in the Recovery Mode Menu).
Please be aware that unless the core service account is not removed first (Apple's iTunes account for iPhone OR your Google account for Android), you will trigger the activation lock (as an anti-theft measure) & require you to log into said account before you can set up the phone. This is something I cannot assist with if you forget your account information, as it places your claim of ownership in question (although I won't accuse you of stealing the phone, but it does raise the question) & will cause you to face a massive uphill battle with the respective companies (as both Apple & Google will throw every piece of red tape at their disposal at you in order for you to assert your claim as the rightful owner).
While it is your choice on what phone you want to go with, I hope you understand the caveats & potential headaches you may face.