Re: Unlocked 830 or ATT version
I believe the AT&T version supports Carrier Aggregation. I think you'll want that if you're on AT&T. It will combine the downlinks for two different frequency bands so you'll have one big pipe for downloading.
It'll help on AT&T. While AT&T has a decent amount of spectrum, their spectrum portfolio is split up between a bunch of different frequency bands, and you can only use one frequency band at a time. So while they usually have enough capacity, their peak download speeds generally aren't as high as Verizon's or T-Mobile's.
For example, let's pretend both AT&T and T-Mobile each have 15 Mhz assigned to the downlink for LTE. Let's also assume they have the same number of customers downloading the same amount of data.T-Mobile has one contiguous block of spectrum, and if you run a speed test, you get 15 Mbps down. AT&T has the same amount of spectrum assigned to LTE, but unfortunately it's split between Band 4 and Band 17. 5 Mhz to Band 4 and 10 Mhz to Band 17. You can only connect to one at a time, so let's say you connect to Band 4, then run a speed test and get 5 Mbps down. Then you switch over to Band 17 and get 10 Mbps down. So AT&T still has the same capacity, and the speeds are still decent (assuming the customers are spread appropriately across the two frequency bands), but your peak download speeds aren't as high as on T-Mobile because you can't access all 15 Mhz at once. Carrier aggregation basically fixes that, so you can access all 15 Mhz of LTE at once. I don't know if it'll add up perfectly (10 Mbps + 5 Mbps = 15 Mbps?), but you'll definitely get better speeds than just using a single frequency by itself. Also, note that this only works for downloads, for uploads you're still limited to only one frequency band at a time.
I made those numbers up, but hopefully you get the point.