830 puts a check mark to a lot of items on the must have list...
Here are the missing features from the Nokia Lumia 730 and Lumia 830 that you may care about | Windows Phone Central - the only advance coming in Denim that 830 seems to miss out on is active listening for "Hey, Cortana" hands free activation of your assistant. All the other future advancements most care about are coming to the device.
However, overall spec leads some to question if 830 is truly a Flagship Device.
What's coming and will it come to AT&T? The only sure thing (already announced) is the HTC One (M8) for Windows Phone.
Many of the advances coming in Denim are Firmware. It's not clear yet what HTC will do to leverage the new API's and if they will follow suit to keep up.
Here is side by side spec 830 vs 1520:
Nokia Lumia 830 vs. Nokia Lumia 1520 - GSMArena.com
or 830 vs One (M8)
Nokia Lumia 830 vs. HTC One (M8) for Windows - GSMArena.com
1520 and One (M8) are
the powerhouse phones in the US for Windows Phone this fall. I discount 930 as it really doesn't apply in the US unless you are on Verizon (which you state you are not) with the Icon (930 variant). You'll see from the spec that 830 sacrifices screen resolution and more tellingly RAM, CPU/GPU chipset and battery to make the pricepoint Microsoft wanted.
If you want true Flagship level and have one purchase satisfy your phone needs through at least this time next year (if not well beyond), I'd say wait for One (M8) or go with a 1520.3.
Personally, having just moved to a 1520.3 from a 925 - it is not too big unless you just HAVE TO use your phone one handed. It's a beautiful beast of a device. Unless you HAVE TO have some AT&T branded app, 1520.3 is better than the AT&T branded 1520 due to automatic 32GB internal storage and Qi wireless charging without a special case. Add in the factory unlocked factor for fast track on getting Denim and it is a big win.
Obviously, I'm biased, but get the 1520.3 yesterday. Don't wait to see what other mediocre thing tomorrow will bring.