This thread has really hit on my current situation.
I've went from iPhone 5 to Nexus 4, and back again about 3 or 4 times over the last few months. Just this morning I returned the iPhone 5 I bought on Wednesday because I've decided its not for me anymore, at least right now, or in reality until the new iPhone is launched sometime this year. IOS 7 will be the dealbreaker for me.
Heres how this has worked for me:
Pros to Nexus 4 (cons of iPhone 5?)
Larger screen makes a mockery of the iPhone 5. Its a great screen, responsive and very clear
Stock Android experience means you should get updates for a while pretty quick from Google
QI charging - yes its a gimmick, but just the ability to set it beside me at night and its fully charged in the morning is very handy
For me - music is important and there are better options on Android - Poweramp and PlayerPro for example
Keyboard - even the stock implementation is so much better than the IOS keyboard on the iPhone. You won't realise this until you use it.
Customisation - a pro and a con for me; I find myself tweaking some things, but its a sinkhole. You can become too anal.
Pros to iPhone 5 (cons to Nexus 4)
Battery life - 4.2.2 is better, but even with this I'm always 'afraid' I'll run out of battery before the evening charge. I've never really had to worry about this on the iPhone - although to be fair any iPhone, not just the 5
Ecosystem - yes there are alternatives, they have been mentioned by others in this thread. But my wife has an iPhone, and with a young son and photostream there really is no other way but Apple. I've now got an iPad mini just for this though.
Much better support - hardware and software from Apple.
So a bit of background if you are interested so you can see where I'm coming from:
Before June 2007, I had a couple of smartphones, the SE P800 / P910 series, but when I seen the Steve Jobs iPhone keynote in 2007, I knew that was the phone for me, so I imported from US to UK in July 2007, and worked my way through all the iterations, even queuing for them at the local Apple store, Belfast. Yes, even at 3am on the iPhone 5 launch date. I've flirted with Android over the years, mostly because of a work issue device, but always felt iPhone was superior experience, so always I was an iPhone guy. When the S3 launched last year, once again my curiosity for Android got the better of me and I bought it - at the time I had a 4S. I found it a great experience, the OS was much more polished and usable and I pretty much switched to it full time. But I still missed the IOS ecosystem to which I'd become accustomed. The S3 was then sold promptly for the iPhone 5 funds.
So I'm telling you all this because it should give an indication of how much I am used to the iPhone ecosystem and how much its became part of my life. I have a lot of Apple stuff in the house, I just found it was what worked best for me.
So finally the Nexus 4 launches late Nov 2012 and I thought I'd give it a blast. It is a very nice device, and I found I preferred the stock Android without all the manufacturers add-on very nice indeed.
So to sum up (sorry for the long post);
If you are really tied to the Apple way, its not impossible to switch. There are ways round everything and for me everything I need is on both devices. Despite that, I find myself hankering after the iPhone, but always lift my Nexus by default. They really are both as good as each other in my opinion. It really does, as ChazUK has said depend on how tied you are to the Apple Ecosystem, and I know in the end thats what is going to drag me back.