I've been agonizing over the same decision.
I had an upgrade from an iPhone 4 on AT&T.
I agonized over all the WP8 phones. The 8x was too low on storage. The 920 was too bulky. The ATIV S might have worked if it ever came stateside.
But eventually I went with an iPhone 5 (and my wife got a 920).
I'll say this about my iPhone: it just works, and it is fast. I'm tired of iOS, I really am, but it's the little things, like universal background audio, car audio connectivity, picture copy/paste, "undo" support, mobile browser site support, multiple synced email folders, the list goes on and on.
I love WP8 for what it does well, but it's still so young. My phone is a tool first and foremost, and while it makes me yawn every time I look at, the iPhone is, for me, a better tool. At least for now.
I also tell myself that if WP8 improves in the next 6 months and/or ATIV S comes stateside, I can always sell my iPhone.