Argentavis only need 175% movement speed to outrun a wyvern but 200% is the safest option since the wyvern breath attacks can still hit you if you're under 200%. Argentavis are also very useful when trying to down another creature, they are decent fighters if leveled properly, and great for gathering runs as well.
Getting a massive TLC update in March 2018. The saddle will act as a mobile smithy, Argentavis will be able to carry with Talons as well as with its beak, reduced carry weight on obsidian and a few other unannounced resources (similar to the mammoth and wood), and will also gain a Rapid HP Regen upon eating (likely from devouring corpses and not specifically food from inventory). Like so people can see!