I will not be exactly pursuing the subject, but just two weeks ago I switched from Lumia 930 to iPhone 6s Plus. I do agree that Windows 10 Mobile proved to be buggy and disappointing, but I'd like to share an opinion about iOS 9, which hopefully will be helpful to some people also considering the switch.
First of all, one can't expect this to be easy. From the very first days of my adventure with iOS I am in constant contact with Apple Support due to a load of issues: Siri not working, iTunes Music Library synchronizing things I don't won't or not syncing the songs I want, regardless of professional help and my best tries. Therefore I do consider both mentioned application as beta quality software. Which struck me very hard - isn't this Apple, shouldn't this "just work"? But no, it is my sincere opinion that Apple has issued an operating system that has two application of poor quality, not very far from worst things in Groove or Cortana a few WM10 compilations ago. So before anyone switches, please consider we are talking of software here and it never seems to be perfect. Even the most revered, I would assume, software from the best performing tech company in the world is just buggy. Sometimes I fight with my playlists or try Siri do something for me and I can only sigh and give up thinking of how many time Groove and Cortana served me better even in, officially, just beta version of the system.
But I don't regret the switch, at least not yet. I am currently overwhelmed with all the apps in iOS ecosystem. You see, all my smartphones were WP phones, and my old corporate iPhone 4 was not supposed to be used to install leisure apps. So having a private and newest iPhone I have access to everything iOS apps can offer and I'm thrilled with the choice. This is not to say that I was missing some apps in WP, since I had everything I needed, but with iOS I can discover a completely new world of apps performing things that my WP apps could never do. At the same time I find some apps, e.g. Here Maps, looking much worse than on WP.
So, not to extend this too much, I'd say that switching is a bitter-sweet experience. More of a eye-opening experience: neither ecosystem is perfect. There is no mobile OS without bugs, and what worked for you in WP might be a nuisance in Android or iOS. I'd suggest to the author of this thread to certainly go for it, but be aware that this expensive device he is going to buy will not necessarily be a solution to all the problems Windows Mobile has.