Android. I've used both and like it or not, you have a better chance of finding a decent phone with phones with Android OS in it.
iPhone is definitely fine, but they're often times over priced. If you DO go with iPhones, go with iPhone 5s. Not only does it only cost $150~ish for a 32GB or maybe even 64GB version if you're lucky, but many apps still works on it and will likely continue to work on it for a while without any issues.
New apps will eventually not work on it anymore and that'll likely be the case by early 2020's. But regardless, at the MOMENT, as an entry level iPhone 5s is definitely a good phone to choose from if you want to go with Apple devices.
As for android, I'm a little less verse, but I'd go after Samsung phones that costs within your budget. Or any Android device regardless within your budget.