I’ve both personal and work. 4 mail accounts. Exchange, gmail and iCloud.
Combined inbox, any folder sync, iCloud and google contact sync without any additional app, easy unsubscribe option for marketing emails, overall layout and aesthetically pleasing look to name a few
I’ve tried outlook - it’s kinda ok. Still have occasional issues with attachments, sometimes stops receiving emails until I kill the app and restart.
Unfortunately Nine Folders doesn't support either iCloud email or Gmail. It's superb for Exchange. I use it for that and for Hotmail/Outlook.com, but use the Gmail client for gmail. All of them feed into the Samsung Calendar and Contacts, or Google Calendar and Google Contacts, so it's only email that I have to use 2 clients for.
I did really enjoy the quick unsubscribe option in the iPhone stock email client, but found it was very lacking in other areas as compared to Nine, MS Outlook for iOS or the stock Gmail app.
I've been using both a Samsung S9+, and an iPhone X for about a week, and overall, I have to say that the differences in productivity, camera, screen quality, etc, are now so small, that I could be happy with either one. I'm debating whether to return the iPhone or keep it and sell the S9+. It's a very difficult decision. I'm traditionally an Android fan...