The highland sword does the exact same damage as greatsword for all attacks except heavy overhead, which does 10 more damage for highland sword, and it does less stab damage than greatsword. It's also much, much slower and has a pretty unusable special attack because of how long the special windup is. Highland sword was mostly fine before the nerf except for it's riposte overhead. There is almost no reason to use it over greatsword right now.
Proof (taken from polehammer.net):