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Maybe something like Mox Opal has them on raw power, but these cards offer broad-spectrum quality. They cross archetypes to power up tons of different decks and don't just fold to one set of hate. Unlike Fatal Push, they are just good against almost everything.
Cantrips and discard are also some of the most difficult cards to play in the format. I say things like "Death's Shadow is easy to play," but that's really "Death's Shadow is easy to play if your job has been playing games with these cards since Bitterblossom was in Standard." I'm a very heuristics-based player, and fortunately for you that lets me put a lot of the tips and tricks for what I know about these cards into easy-to-digest snippets.
Most of the examples here will be in context of Grixis Death's Shadow, half because that deck plays both of these types of cards and half because that deck is still insane. I'll try to touch on spots where that might differ for cantrips in combo or discard in…uh…a different attrition deck.
Modern cantrips aren't quite Brainstorm in terms of power or skill required, but they are still cantrips. Card selection and selection of card selection takes a bit of finesse.
Which One Belongs?
In general, Serum Visions is the most powerful card of the group. It is pretty straightforwardly the best selection. Thought Scour puts a random card into your hand, Opt lets you choose a known card or the mystery box, Sleight of Hand is the same two-card choice with an open mystery box, and Serum Visions lets you keep both or neither of the two cards you see. Having more options is just better.
Serum Visions does make you wait for the selected cards relative to Sleight of Hand and Opt. Modern isn't that fast right now, but there could be a future where your deck plans on comboing off after leading a turn on a cantrip where Serum Visions being slower matters. It involves some Legacy Ad Nauseam Tendrils-level nonsense, but it could happen.
Example: A dedicated Empty the Warrens deck might want Sleight of Hand and Opt over Serum Visions to hit more mana that turn. See this heavily banned Jeremy Neeman list that might have been the most busted deck at the first super-busted Modern Pro Tour in 2011.
Sleight of Hand verus Opt is just a "sorcery versus instant" question. Sleight of Hand gives you more choice by seeing the second card, so you need a real reason to Opt. You don't need a big reason, just a real one: Snapcaster Mage alongside removal, countermagic, and so on.
Thought Scour is almost exclusively used alongside delve creatures as a cantrip delve Ritual. Just adding Snapcaster Mage fuel or getting lucky and hitting Lingering Souls isn't enough, but once you go down this road, having those bonuses is nice.
Bedlam Reveler isn't technically a delve card, but it operates in a very similar fashion with Thought Scour. The pure Mardu lists of this deck have floated around for the last few months, but Chase's list looks like it could be a real step forward for the archetype.
When Do You Cast Cantrips?
Example: Your opponent mulligans on the play, scries to top, and plays a tapped Temple Garden. Thought Scour them either hits something really important like Collected Company or Devoted Druid or hits a land they are counting on drawing. The same applies to Serum Visions or unfortunate Lantern of Insight players.
The rest depends on how specific your deck is. Combo decks typically cantrip as soon as possible, but fair decks may want to wait to choose what they want to draw after an alternate one drop.
Example: This is one reason Grixis Death's Shadow is so great: you can use your mana on Turn 1 to Inquisition of Kozilek and then Serum Visions on Turn 2. Jeskai and other slower decks use Celestial Colonnade or fetching a tapped land as the one-drops.