You wont go wrong with a One, I have one, as I do a iPhone 5, and as of today a GS4 (its my job).
Apple wont change IOS so much that it will make it a new OS, and thats of course what you will get with the One, but consider this.
Apps, I have been in Android since the G1, and I’m one of the minority who will spend money on Android Apps, but they are not in the same league as iOS.
Google, yes of course Android is better integrated with Google, but its not like iOS is Google free, it makes some great apps for iOS, so you miss very little of its advantages on iOS (At the moment).
The One is not on the latest version of Android. Thats ok because 4.1 and 4.2 are not too far apart, but its worth noting that as with all Android devices (outside the nexus range) you run the risk of your £500 super phone never getting an update.
If I break my iPhone (or have an issue), I can get it fixed or replaced at an Apple Store close by. What are your options is the One breaks.
Unless you are going for a Dev phone, you will have to start jumping through hoops to get bootloaders unlocked, custom recovery installed and rooting initiated (assuming you want the full Android experience). Its possible, but its not for the faint hearted (fastboot, ADB, RUU and the like will be your new friends). Thats and XDA Developers :-)
Peripherals…. What Apple lacks in number of device options is makes up for in sheer weight of add ons. Its worth considering.