Most folks prefer to overlap because it's easier and stronger. One problem (that others have pointed out) is that you'll typically trap air and water and rust build up and work its way out. Another problem not mentioned is that if this is a body panel, you'll find it anywhere from hard to impossible to shape the overlap joint after it's welded. This means that if the any warpage occurs while installing (and some will), you'll have to add even more body filler since your hammer/dolly/shrinking disk/etc will do squat.

A butt weld is harder to do but worth the effort. Five years from now, when your paint job starts bubbling up around the overlap joint, you'll look back and wish you had done a butt weld.

Thanks,
PeterM