I'm not sure yet as I'm waiting to see what new features will be released in iOS6. As of right now, I'm torn between the iPhone 5 and one of the new WP8 devices that will be released later this year. I've grown tired of Android, even with the release of the Galaxy S III and the Galaxy Nexus. I'm looking for something that runs smoothly and has great battery life as well as LTE. I like the idea of the iPhone 5 only being slightly larger than the 4S as the huge Android phones are bigger than I'd like, not to mention the fact that even stock Android can lag big time. As far as I'm concerned, it would take a Nexus device from Motorola to get me to consider Android again.
Depending on what is revealed about WP8, I think that will be good competition for iOS. While WP7 may be lacking a little bit on the features, it still runs extremely smoothly without bleeding edge tech. In my time with it, WP7 was fun to use and with the new basis for WP8, things should run very well and have a lot more functionality. All I can say is that with all of the different platforms out there, it's a great time to be in the market for a new phone.