If you're playing casually, then it doesn't really matter. The gnome racial helps with enemies kiting you in PvP and in some situational PvE stuff but again the differences are so minimal it really shouldn't be the determining factor. Humans with swords are going to get the extra skill which is a significant DPS increase for raiding. But for leveling, it's barely any difference as a warrior since you shouldn't ever be attacking anything above your level anyway.