The second shot comes from the midpoint of Aliens; Having driven the armoured vehicle into the compound to rescue the marines, Ripley is now desperately trying to pilot them to safety. When a Xenomorph leaps on to the top of the vehicle, Ripley slams on the breaks and sends it flying forward, then accelerates, crushing the creature under the wheel. The moment is sold as fairly incidental kill, but in terms of Ripley's character development it is an essential one. She has finally killed one of them, and the look on her face - one of absolute determination - tells you everything you need to know.
And while tonally the films are significantly different – one is a horror film, the other an action adventure – watching them back to back makes you realise that Ripley's evolution over the two films feels completely natural (and therefore, so does the tonal shift). That, I think, is why these two shots stuck out in my brain. One is of a terrified final girl, the other of an arse-kicking action heroine (something also reflected in the lighting of the shots - Scott employs rapidly strobing white light, while Cameron uses an "inside of a submarine" red). Aliens is such a phenomenal sequel because it is driven by the need to gives it central character closure, and Sigourney Weaver makes you believe she gets it.