The tooltips for event only list rewards, and do not give the traits that your society will permanently receive as defined in trait_society.xml .
It appears the rewards are (first the tooltip, then the trait):
magic: +2 relics, magic_mod+5
ancestors: +10 ancestor power, diplomacy/magic/martial/trickery all +1
infantry: +4 infantry, marital+3
explore: +1000pop, explore_mod+10
The tooltip rewards are pretty minor. I have no idea what relics are worth, or how they are used in the game. I've got the "Ancestor Power" button on all the time, and it appears to do nothing. +1000pop only costs 1g once the game starts. +4 infantry is worth about 60g, so that is the one I always chose before looking at the data directory.
Looking at the traits, the magic and war mods give a +3, while the ancestor gives 1+1+1+1, which makes it "better". Exploring gives +10, but again I have no idea what that means.
I think you need to communicate on this screen that the choice will give you a permanent societal buff. You might as well put it in the tooltip, because it will show up in the "Society" screen once the game starts.