Here is more explanation on the issue.

TL;DR: If you use return and func() returns false, the form will not be submitted. Without return, it will run the function and submit the form regardless of the return value of func().

You are better off to have something like this

<form id="submit-btn">

Then for the javascript

document.getElementById("submit-btn").addEventListener("click", func);

function func() {
//Stuff Here
}

It is just cleaner, but as for your question. Really they do perform the same thing. It just depends if the output of your function is a boolean value (true or false) if you type return func() and it is a false value then the form will not submit. If you type func() then no matter the output the form will be submitted.