The FitGirl repack is technically superior because it uses the newer 1.10 version. However, the -R.G. Mechanics- v1.5 is historically significant and sometimes preferred for extreme low-end PCs because its crack is "lighter" on CPU overhead.
Post-patch reports from players noted a transformative effect. One player on NeoGAF stated, "The 1.5 patch definitely improved performance for my 980... I can finally play... stutter free!". The patch wasn't perfect for everyone, but it was the turning point where Far Cry 4 was widely considered "ready for release". -R.G. Mechanics- Far Cry 4 Version 1.5