You need to tell us, very explicitly, what you want in your device. The 920 is worth the upgrade from the 820 in my mind, as it (until 2/21, I think) comes with a free wireless charger on AT&T, the camera is superb, and getting 32 GB of built-in memory is worth it. If you don't care about added storage (or you're fine with the SD card route) and you don't care about the camera or display quality, then the 820 would make sense.
If you want specific non-WP apps like Instagram, then the Nexus 4 is the way to go. I personally went 920 because I've got 25+ GB of stuff on my phone (almost 18 GB of music alone), I use the wireless charging CONSTANTLY, and I enjoy playing with the camera.