I have both the 920 and the 4S. The core functionality (calls, emails, texting, camera, etc) I find to be a lot better on the 920. I also like using WP8's interface a lot more. I'd get dropped calls on AT&T with the 4S all the time, haven't had one on AT&T with the 920. Voice quality is clearer also, especially through BT in my car. The 920 is noticeably larger and heavier, but fits my hand better and makes for a larger keyboard that I find easier to type on. I #$%@ing HATE iOS' autocorrect, but I love WP8s - it's not too intrusive and usually ends up correcting the right things. The 920 is also sturdier (I dropped mine yesterday from 3ft onto concrete and it's fine except for a few chips in the polycarbonate). I've had zero hardware issues with my 920 (no overheating, no camera wonkiness, no reboots, nothing), and I'd consider it my favorite phone I've ever owned. Oh yeah, and the color choices are really nice. I got black because I'm into photography and wanted the phone to be as low-key as possible, but I love the Cyan and Yellow.
That being said, the 4S has some advantages. iOS is a lot more mature, so you'll get things like a notification center and separate volume controls for ringers, music, etc. The stock music player on the iPhone is a lot better, but I use Spotify (now available on WP8), and that works well enough (still better on iOS, though). There are far more high-quality apps written for it (especially games), and just about anyone writing an app nowadays will write it for iOS and Android first, and then, y'know, maybe get around to a WP8 version at some point. We'll see. This is changing, but slowly. iOS has Google apps including Maps and G+. WP8 does not, and probably will not... like ever. Office is the only "app" on WP8 that's better and more functional than anything you can find on iOS. You're still considered an early adopter if you get a WP8 device right now. The iPhone also has a lot of accessories made for it if you care about that sort of thing. Every bag you buy that has a phone slot will fit an iPhone... some may not fit the Nokia.
The 4S is more versatile and supported, for sure, but the 920 is a better phone IMHO. I'd go to an AT&T store and handle both of them and play around w/ the OS a bit to get a feel for it.