What are the pros and cons of emitting events vs return statements?

Question

According to this answer the return data of a smart contract function call can be accessed, however the same return data can be collated into an event to emit, so when should either method of returning data be used and what are the overall pros and cons of each including gas considerations?

// method 1:
struct Data {
  uint id;
  int val;
}

function returnData() external returns(Data memory);

// method 2:
event Data(uint indexed id, int val);

function emitData() external;

Answer 1

Let me elaborate a bit on the valid answer from @pbsh.

As mentioned, they behave quite differently and are for different use case. Events can't be monitored by other smart contracts, only by external systems monitoring the blockchain (and the specific contract). Partially the opposite is true for return values.

What I mean by 'partially' is that an external system can't directly access the (possible) return values of a transaction. The typical solution is to emit events with required data and catch those when the transaction is processed. However, if a static call (not a transaction) is used, you have access to the return values.

As for gas considerations, return values are actually cheaper to use than events. In the following test contract executing testReturn costs 21401 gas and executing testEmit costs 22506 gas.

Example code:

pragma solidity ^0.8.0;

contract GasTest {

  event A(uint256);
  
  function testReturn() public returns (uint256) {
    return 5;
  }

  function testEmit() public  {
    emit A(5);
  }
}

Answer 2

When you return data from a function, the return value can be accessed only if you know the transaction hash in which the function was called. This is because the return value is scoped to that particular transaction. However, you can set up a listener that can be always listening to certain events. And if you want to get all the events emitted by the contract, you can query them. But event logs are not visible to other contacts. If you want to perform some actions based on the return value of some other function, you won't be able to use events. Think of events like console.log() and return as return value of functions.