Everything is context sensitive but there are a few principles that tend to hold up.
Decks that draw themselves (engines) tend to be better because they draw newly bought cards faster, which lets them improve themselves faster. The limit on how big to build them depends on context for how fast you can improve your deck (how good the draw is). Expansions tend to push towards engines because playing engines is fun.
Trashing is a good way to make a deck that draws itself since you'll have fewer cards to draw. Trashing in Dominion usually makes your current turn worse and future turns better, and in practice, this is usually worth it, but it can depend. Sometimes the draw is really good and then trashing matters less. Sometimes the trashing is too slow and then you don't want to trash as much. "Trashing a junk card = gaining a draw card" is not really accurate, but is approximately close enough to keep in mind.
Dominion matches are very rarely about "my opponent did A, I will do B to counter it." Most of the time it's "A is the best thing to do, how can I do it best?". Questions like "is draw better than trashing" aren't really helpful because the answer usually turns into "does it let you do A better?"
The Dan Brooks "20 questions" chart still holds up pretty well IMO. Usually the important parts of a kingdom are "how many cards can I gain per turn" and "how much can I draw here"