When you say stationary do you mean parked, or just stopped at a set of traffic lights for example?
If the latter, then I don't use either, just keep foot on the brakes.
Park locks up the transmission, so I'd imagine in an impact that it would put quite a lot of strain on the gearbox? In terms of how far you're pushed forwards, I still think the actual brakes would be the best at holding you in position.