Hey Dan!
It totally depends on what you’re trying to do!
EQ and Compression are powerful in both their own cases.. and are often most powerful when used together 🙂
Normal EQ is just “static”. It can remove or boost your selected frequencies, and that choice remains whenever the audio plays through that EQ (unless you automate the EQ’s bands).
Compression more effects the volume of a sound. However, it can change the frequency balance, too, but not like an EQ would.
In your case of “sidechaining reverb”, that is just allowing to make space, and be used as either a mixing technique, or a beatmaking technique, to help the instrument stand out, then the reverb (or delay) will be audible once the dry signal (instrument) stops playing.
If you use Sidechain Compression on a Reverb Send, that is allowing space in the mix by removing the whole audio signal while the dry signal plays (this is known as “ducking”.. or turning down the volume).
If you use EQ instead of Sidechain Compression on that Reverb Send, you’d just be removing/boosting certain frequencies, but the other frequencies would still remain audible, and not be as prominent as Sidechain Compression!
I like to think of EQ to help thinks fit, and Compression to make things remain consistent. (However, those rules don’t fully apply if you’re being aggressive and using EQ/Compression for Sound Design purposes!)
For those reading, you can learn more about EQ/Compression in these courses:
