For me the best plans are Google Fi or T-Mobile one with the international add-on. Internationally, they are the same in terms of coverage for practical purposes. Domestically, Google Fi adds Sprint and US Cellular's coverage. Google Fi also does some good Wi-Fi roaming. For data while abroad, Fi can be faster, recommend the $15/month add-on for T-Mobile for faster data and unlimited GoGo on flights. While Fi is cheap when you use little data, the data costs can add up quickly to take you to $80/month+tax on a single line. T-Mobile's prices (they also offer military discount) are inclusive of taxes and fees.
Why all this about plans? Because Fi really needs one of their devices to work well. T-Mobile you can take your pick from Samsung, Android, iPhone, etc... T-Mobile also has some subsidies on phones. If you generally use 1 GB a month of data or less, Fi is cheap and a clear winner in price.
Security. SMS is probably the most insecure technology we use day to day. Leaving iMessage for Android means you will now use SMS to talk to other iPhone users. Work around: get WhatsApp or Signal (your contacts need to do this too) -- of the widely used messaging platforms these are the most secure. As far as Phone security goes I would rate it as iPhone, Pixel, and then all the other Android devices. iPhone does better sandboxing of Apps (meaning its much harder for that random weather app to read data from your other apps). Pixel/iPhone both get updated fairly fast, while Samsung and others are flapping in the breeze for months to a year. Android 9 "Pie" was released on August 6th. Currently no Samsung Note devices run it. Android 8 "Oreo" released last year is only on 2017 and newer Notes. Android devices get orphaned fast. iOS 12 when it drops in a few days will run on everything from the 2013 iPhone 5s to the new Xs phones. So if you plan on keeping the device a few years -- there's that.
Customer service. I was abroad and smashed my iPhone. Screen was unresponsive so I used iCloud to remote wipe the device. I walked into an Apple store and walked out in about 30 minutes with a replacement phone thanks to my Apple Care. I had three more weeks in that trip that would have meant getting a burner phone or something if I had to send my device off somewhere.
Interface: Android wins. iOS was designed to be easy. But now with triple home button taps, peeks and pops, settings in the app vs settings in the settings menu, and generally subpar multitasking it can be infuriating sometimes. Each release of iOS and iPhone since the iPhone 4 came out feels iterative of their last phone and derivative from their competition.
I currently run an iPhone with my primary number. This compliments my iPad as they both run ForeFlight. Bang out a flight plan on my phone while sitting in a Cafe, see it on my iPad when I pull it out of the bag at the aircraft. For my GA flying I get briefs and file on either of them. I have a secondary Pixel that I use for Android Auto as it has Google Maps. Apple CarPlay only supports Apple Maps for now -- iOS 12 is supposed to bring Google Maps/Waze support at some point. Apple maps doesn't work in many of the places I visit. That, combined with the ability of the new iPhones to simultaneously use two SIMs means I can have my main number, and a burner phone number for craigslist or host nation data plan or whatever kids do with their second phone number these days. So in a few months I will probably be iPhone only despite it being the inferior product due to the reasons listed above.