AT&T don't have wireless charging built in and are 16gb not 32gb
If you're going w Black, AT&T has a black 32gb version (but still no wireless charging, I miss that). It does support a charging cover (xtra purchase) with a different charging standard (other, older Lumia's on QI wireless charging). The newest Dev Preview (WinPhone 8.1) allows for install of apps to sdCard & supports up to 64gb cards (
probably 128gb, but not spec'd that way) so onboard storage is less of an issue.
Other difference, AT&T Lumia 1520 is NO Contract priced @ $550 on Microsoft store (w 5 - 10% OFF coupons floating around, online), $585 from AT&T. Int'l unlocked version will cost (@ least) $100 more. I got my Lumia 1520 off eBay for $425 (+ $5.95 shipping) NIB, sealed w warranty (but, I performed my due diligence on seller, before I pulled the trigger & used Paypal, JIC).
Int'l device warranties can be troublesome, these devices were NOT meant for the US market.
EDIT: I believe the AT&T version has an xtra (different) band(s) for LTE, just in case you decided to switch. Check the Nokia site for differences (you may have to change regions) or google the different versions (I believe there are 3 different versions - by RMxxx, where xxx is 937, 938 or 940 (AT&T version))