Beastmaster feat Beastmaster Bond vs Mythic feat Telepathic Union.
1) Access.
Mythic is uncommon. You can only get Telepathic Union in a Mythic game. Beastmaster is Common.
Beastmaster is an archetype though. It takes two feats to get Beastmaster Bond and a third if you want any other archetype feats from a different archetype. A Druid character in a Mythic game without Free Archetype may want to only have Telepathic Union instead of paying three class feats for it - at least via retraining once they hit level 14 to get those class feats back.
2) Shared Language.
Beastmaster Bond gives the communication range, but doesn't augment the communication in any way. The Animal Companion is still an animal and communicates like one.
Telepathic Union gives the telepathic communication at a mile range and as though you are speaking a shared language. Though, that shared language part is given from the Beast Lord Dedication rather than something new with Telepathic Union. Still, it isn't something that you are going to get from the Beastmaster archetype.