Another way to look at it is just to consider the difference between the effects to yourself vs. the difference between the effects to your opponents.
The difference in the effect to yourself is a terminal +1 card draw. This is significant: the difference between Moat without its defensive capability and a Smithy. That extra card you draw is worth roughly the average treasure value of all the cards in your deck. Early on, that's $0.7 and usually rises quickly to over $1. So let's say it's $1 on average. Well, an extra $1 is the difference between Copper and Silver, or Silver and Gold.
However -- the fact that it's a terminal draw either way means that the extra card makes less of a difference than it does in a non-terminal. That is, the difference isn't as great as the difference between a vanilla cantrip and a Laboratory.
But now look at the effect on your opponents. Council Room's free card to your opponents is a non-terminal draw, because they don't use an action to get it. Not only does Margrave deprive them of a free Laboratory effect, it attacks them as well!
I think what I'm saying is that if you had a $5 card that said, simply, "+3 Cards, +1 Buy" and nothing else, it would be normally preferable to Council Room!
However, there are situations where these differences matter more or less to you or your opponents anyway. Times when you might prefer Council Room:
(1) The obvious one is if you've got an engine that lets you play Militia or Ghost Ship or something after a Council Room. Then Council Room is obviously better, because you cancel out the advantage to your opponents while reaping the advantage to yourself.
(2) If your opponent is reliably drawing his deck every turn, even when he has to discard down to 3, obviously prefer Council Rooms.
(3) If your opponent has a "draw-up-to" thing going on, like a Festival/Library engine, neither the Laboratory effect nor the discard attack will affect your opponent's fortunes as much as usual. True, a free Lab effect might cause the engine to fire on a turn when it otherwise wouldn't, but very often the free Lab will just mean that his first Library draws one fewer card, or that the discard attack means that his first Library will draw two more. Big deal.
(4) If you've got a +Actions/+Cards engine with a non-drawing Village, you might need the Council Room to compensate. Just as Village/Moat (+1 Card/+2 Cards) doesn't work as a drawing engine under normal circumstances, you'll have a hard time getting a Shanty Town/Margrave (+0 Cards/+3 Cards) engine to fire. Harder, actually, because although one play of each draws the same number of cards, the former gives you an extra chance to get the drawer into your hand.
But a Shanty Town/Council Room (+0 Cards/+4 Cards) engine can work, because the extra card from the Council Room mostly compensates for the lack of draw on the Village component. Now, a "pure" Shanty Town/Council Room engine probably doesn't outperform a simple BM+Council Room deck, but since we're only talking about boards that have both Council Room and Margrave on it, you can toss a single Margrave into that Council Room engine, play it last, and thereby (roughly) negate Council Room's penalty. This trick should work with any kind of Village, not just non-drawing ones. So here's one case where you want both cards, but mostly Council Rooms.
