Well based off the spec page on LG's website it has the bands GSM 850/900/1800/1900 MHz, UTMS I/II/IV/V, LTE 2, 4, 17*. So it looks like to me the only thing that is different from the ATT model is there is no 850Mhz LTE and it has AWS HSPA+ capabilities instead. Att thus far only uses 850Mhz LTE in one market which is Bloomington-Normal, IL. It has their band 17 700Mhz which is by a large margin their most common and it has Band 4 AWS LTE which is their second most common and it even has Band 2 1900Mhz PCS LTE which they plan to use eventually. So it will work on 99.9% of ATT's current network perfectly including LTE except in Bloomington it wont pick up the 850 LTE. In the future ATT plans to launch more 850 LTE but that is a year at least away so if you find a good deal on the Tmobile version you will be fine except in a very few isolated areas of the country.
Yes, it should work fine. I'm kinda jealous of that fact too because an unlocked T-mobile version will work great on AT&T, however an unlocked AT&T phone will be poor on T-Mo.