Really, I'm not even sure it's a tough one. The way I resolve timing issues in a lot of games like this is like this:
When an effect is triggered, take a mental snapshot of your board and your cards. Anything in play at the instant that the effect is triggered will be processed. You have the building that gives you a card, so you get a card. The card that you gain was not in your personal supply when the event happened, so it won't be processed.
So far, this way of thinking about timing issues hasn't led me astray. (:
In a normal game I would agree, but with this game I've been surprised a few times by it's..... permissiveness. Especially for example that a minion's benefit can trigger a card with "Whenever you use a building to...." effects, where the wording really sounds like the building has to be the thing providing the benefit to trigger the card effect.
Plus I wasn't sure if there was a rule I'd missed about whether building income happens before Guest income, or vice-versa.