I thought you meant mill 3 vs draw 1. Thought Scour mills 2 + draws 1. Tome Scour mills 2 + mills 3. The difference between them is mill 3 vs draw 1. The sentence started with "while Tome Scour puts", so I thought you were still comparing it to Tome Scour and assumed the 3 came from that difference. The card in hand isn't better than 3 more milled in that scenario.
I wasn't arguing anything about a card that only mills 3, because we aren't considering cards that do that. +1 card in hand is better than +1 card in the yard, obviously, but there aren't interchangeable cards that offer that tradeoff. The options are:
Thought Scour,
Tome Scour, or maybe some other cantrip like
Preordain.
Slippery slope fallacy. It's one card, not every draw step.
Over the 1 card I disagree with you, but acknowledge this is a more controversial point.
More cards are better. The question is if 1 card makes a difference winning games that we couldn't have otherwise won vs the win benefit offered by the other spell. It's not just "replaces itself vs doesn't" but "replaces itself vs mills 2/replaces a combo piece". That's more difficult than it seems and I think it deserves more testing.
Cards matter. But card selection matters more. Most of our other spells provide card selection (Brainstorm, Ponder, Preordain, ETutor, Burning Wish, Intuition, even Gamble). We can use them to dig into better cards and also improve our next draw step. Because of all the cantrips, tutors, and fetches in some lists, almost every draw is sculpted. We aren't at the mercy of random topdecks until midgame after disruption. When we are topdecking, we topdeck much worse than fair decks because we need to sculpt specific combinations of cards.
Thought Scour gives no control over what you draw, and you can't even use other cantrips to set up the card that will go to hand.
Manamorphose was like that. Getting a card back is good, but without card selection it was rarely drawing what was needed while taking up a spot of a card that could provide selection just to marginally fill the graveyard.
I cut my Thought Scours for
Preordain, which digs deeper with more card selection, but doesn't mill. Overall it gave me more consistency.
As for Thought Scour vs Tome Scour, there are situational differences. What are you digging for? Thought Scour is worse at digging for BF or LED. Tome Scour replaces BF and digs for LED better. Thought Scour gives you the chance to dig for other cards that need to be in hand, like Breach, protection, or lands, but it also gives no card selection and fuels the grave less.
I think there are merits to both and it deserves testing. Cire has good results with Tome Scour.
If you're in that position, have you ever tried escaping Ponder? I've "gotten there" before by chaining Brainstorms and Ponders into gas. You get the same benefit of putting cards in hand and getting to cast them, only with more choice.