What is more important, stopping power or shot placement?

Shot placement.

Stopping power doesn't mean anything if you don't hit what you are trying to stop.

That said-- it depends. If you are talking about laying down fire in a spray-and-pray style repulsion of an oncoming banzai-style attack of the zombie apocalypse, stopping power is likely more important. If you are trying to snipe the bad-guy who is trying to shield himself with innocents, shot placement is exponentially more important.

On a more realistic example: if you are trying to take a deer for the trophy/ taxidermy so don't want to damage the bust/ head area but don't mind if it runs a bit: shot placement. If you are trying to drop it so as to not spoil the meat with lactic acid: stopping power (assuming you still have good enough shot placement to actually hit it.)

Bottom line: with 'issued' gear, only one of those things lies within your immediate control.
If you are talking about personal gear, you don't have to compromise as much.

They’re both important. If I had to pick one, I’d go with shot placement.