When dealing with discards and a cantrip in hand you have to consider what the cantrip might be and how likely that is to synergize with what you have.
For instance go back to the hand of treasury x3, goons, council room and you've just been goonsed.
Your options are:
1. Keep the treasuries.
2. Keep the CR, G, and a T.
3. Keep 2T and CR
4. Keep G and 2T.
Now let's say you have 3 possible draw items: village, estate and silver. When paired with treasury these become equivalent to: bazaar, copper, and gold.
If you had a hand of bazaar, copper, gold, goons, CR; most of use would keep the bazaar, CR, and goons (maybe the gold near game end). However we don't know which order we will draw so we can look at the above cases as follows:
1. You draw one card live and have 5 coin.
2. You have a 1/3rd chance of hitting a live CR and following it up (eventually) with goons, you have 1/3 chance of being able to play goons with 5 coins (assuming that goons is better than CR) and a 1/3 chance of hitting goons with 3 coin.
3. You have a 2/3 chance of hitting CR live with at least 2 coin. You have a 1/3 chance of hitting CR dead with 4 coin.
4. You have a 2/3 chance of playing goons with 4 coins and a 1/3 chance of hitting goons with 6 coins.
So because you lack information, you can't hit the extremely high payout every time of having an assured CR/Goons hit. This makes treasury weaker than it otherwise would be ... I just thing these sorts of concerns are mitigated by the fact that cantrips are very useful in engines.
On SP engines: Pulling the cards with SP can be better in some of cases:
1. SP can only draw actions, treasury can draw non-actions. If you can set up to more of your non-actions with cantrips and all your actions with SP, you maximize effect. If you draw actions with treasury, you just wasted draw (though this is often unavoidable).
2. TR/KC. If you need the village effect, draw, or whatever from the treasury then it is much easier to hit that if you pull them with SP rather than play them out first.
3. Reuse of cards. SP -> draw a bunch of treasuries and two cellars -> cellar treasuries -> draw coin/VP -> cellar a VP -> draw a treasury -> play and draw through treasuries. Also secret chamber, village, SP, can rediscard many times for cash.