Either rail is powerful enough to run your card. Since Antec provided you with a convenient double cable ( 2 X 6/8pin connector ) you may as well use one to keep your cables neater as one is capable of approximately 372 watts. If you ever get a second GPU for SLI, then it is important to choose plugs on the PS that are on the other rail ( +12v2 ).
Antec was kind enough to put descriptions of the rails on the modular panel, but you had better take a look at the table and the text below to figure out the optimal sockets for all the PCIe cables.
12V1 - Peripheral, left 16-pin socket
12V2 - EPS, ATX, right 16-pin socket
Even connecting all modular cables to the HCG-750M will leave one of the two 16-pin sockets vacant. You had better connect both PCIe cables to the left 16-pin socket because the 12V2 rail feeds the EPS connector; that is, if you don't want to mix them up, which could trigger the unit's OCP. Taking into consideration the only two +12V rails, we think that power distribution is good, but users should follow the aforementioned advice to avoid any problems.