As far as readability goes, the early-return style is the better approach. Just add a blank line for better separation. I additionally like to keep things consistent, so I also always add curly brackets to ifs and a space after the keyword, but that's purely my taste :)
Talking performance, in your case, the early return might safe further if branches later on, so it might be more performant. If you have to use an else-branch, always try to put the more common case into the truthy statement, as most interpreters and compilers optimize for that.
// Example:
function doSth(a,b) {
if (typeof b !== 'number') {
return 0;
}
// the following branch is not evaluated if b is not a number
if (a > b) { // <- likely branch for app-specific use-case
return a * b;
}
else {
return a;
}
}