For many reasons (stability, sweatiness, not landing on stuff when you crash), low down on the bike seems the obvious answer.
However, because it is fixed, it is inert so you cannot use it to your advantage and it could be a disadvantage if it's in the wrong place for a given situation - heavy panniers at the back can make the front wheel light, a heavy saddle pack lifts the bike's c of g way over the axles etc. Weight on the rider, however, can by used advantageously and shouldn't act against you in a moment of adversity or even when climbing.
However, for the weight of a few tools etc, it will make little difference either way.
I'd put it, especially a pump, onto the bike as those things could punch a long way into soft flesh if you land on your back in an accident.