Yes of course this discussion only exists if you actually want a KC in your deck. If there's nothing good to KC, you obviously take gold with your $7. But I would posit that if you have a terminal silver action, you'd still take the KC here - provided you are going to get more KC later in the game, and there are other actions to take later to compliment the KC.
My interpretation of the question was that he was looking if it was better to get KC with low action count, or if he should get more actions first. I would always take the KC here, provided I was going for a KC deck, even if I had a decent chance of it being blank on the next shuffle. It's a lot easier to pick up lower-cost actions on subsequent turns than it is to pick up a quick KC. And while a blank KC stinks, it's not THAT bad, and if you get the chance to get an early KC'd action (like say turn 4 or 5), that has the potential to instantly win the game for you (pretty much).
For example, for the Swindler/Silver opening on a kingdom with Lab/KC. Buying a lab with $7 will help you draw your actions more, and could help you get a KC again. But I'd personally rather get the quick KC. If I miss the KC/Swindler on the next shuffle, no biggie, I can grab another swindler at the very least (I could get EEECC, but still...). However, if I hit a turn 4, 5 or 6 KC'd swindler, it puts my opponent in a really bad place.
Picking up a Lab in that spot, I could definitely hit a bunch of $6's in the next turns, which isn't terrible, but I think I'd rather the KC here. Also, I think KC and any terminal silver is pretty solid, because all you need is a copper in the same hand and you get a 2nd KC.
Now I think that's a more interesting question. Suppose you do pick up the KC on turn 3 with $7 and a terminal silver action in your deck. If you hit $7 again, do you pick up a 2nd KC with only one other action in your deck or do you get another action, say the lab. I would pick up the lab on this $7, but I could be wrong there too?