The reality of never seeing that person alive again is what would make that decision very difficult for me personally. Knowing that they could never talk, move, act like they were before the accident is devastating in itself. If I were in that situation I know I would want someone to end my life, even if it's painful for them, just let me move on you know?
As for me having to make that decision, well I know it would be painful, but I also know that there is little to no hope of recovery(At this time anyways). I would do it, but it doesn't mean I have to like myself for it.