Beastkage barely took any damage from KCM FRS, and that's an attack that had the elemental advantage over his cloak. I don't think Afternoon tiger is significantly stronger than KCM FRS, so it's probably not going to do any damage to him. However, Beastkage becomes a lot less durable when his Raiton cloak gets blown off, so if Afternoon tiger can blow the cloak off with the first hit, I think Gai can inflict major damage with a second tiger if he fires it immediately after.
So I'd say it depends on that. Gai however has never been shown to be able to use double AT, just using one of them seems to be strenuous on his body, so I'd side with Sandaime raikage winning more than not.