Hard for me to say without using it. I use an iPhone as my daily driver, but I've got a Lumia 920/1020 and a Galaxy S3 sitting around. I really want to like and use Windows Phones, but I'm just not sure yet. I also wish there was a smaller high end phone. I like smaller phones, but anything with a 4 inch screen is low or mid end.
iOS has the functionality and apps I need, but I do wish there were a couple tweaks for it such as a modifiable keyboard. Siri is also pretty useless IMO. I like that Cortana can be used with typing input as opposed to only voice. I also need to know if WP 8.1 supports .gif in text messages and allows me to hide the actual message a notification gives. I send and receive a ton of .gifs and I don't like that if someone texts me, it displays the message for everyone to see without even opening it. I've heard IE11 brings some much needed improvement as well. If my minor annoyances have been fixed, I could see myself moving to it.