But if it's 10% of other faction income, will the stronger one agree to this? mostly they won't, unless they have other war with other faction of equal or stronger strength, in this case they perhaps want to agree to this to boost their income and to lock you in peace with them so they can concentrate on their real opponent, but this case is rare, and even rarer is how many player pay attention to diplomacy in this game, a sad fact.
While i mostly agree with economic one, sometimes you can be surprised that it can often beat trade income wise. This usually happen if capital city is near river or there is a gold mine or material mine nearby or whether the town has high essence (material influence gildar income, and there is a spell that increase city material depending on its essence yield), all of this variable will insanely boost gildar income, your capital can even reach 50-100 gildar income / turn with normal tax, most economic treaty usually give 15%-25% bonus from that, can be higher than that in huge map.
I don't know what influence AI decision about these two treaties, i guess only brad know this because he is the AI expert here.
But from my experience AI ask for economic treaty more than trade treaty.