The correct one is PACKAGE_PIN. If you do a "report_property" on the port you will see that it has both a LOC and a PACKAGE_PIN property. The LOC constraint is the location on the die, as in IOB_XnnYmm - its grid coordinates. The PACKAGE_PIN property is the name of the package pin to which it is connected. So the correct one is the PACKAGE_PIN.
However, both in UCF as well as in the earliest versions of Vivado, the LOC property was used. So, when you apply a LOC property to a port, it actually cheats, and assigns the value to the PACKAGE_PIN property instead. As a result, the net result is exactly the same if you assign the LOC or PACKAGE_PIN property - but PACKAGE_PIN is the correct one, and in some future version of the tool it may no longer accept LOC in place of PACKAGE_PIN.
