I played custom game to see what is true.
(Do note: These pictures are outdated. Bonus gold is now +3 )
You get 3 gold per kill and 10% buff to AD and AP.
Also, every time you capture an altar, you gain 80 gold per player, 240 gold total for the whole team. This means capturing both altars will net your entire team 480 gold.
You get three extra gold per minion kill. Additionally, holding both gives your team a 10% buff to AD and AP.
Capturing an altar makes it uncapturable by the other team for 90 seconds.
Lastly, your team is granted vision of the altar itself.