Interesting.

IMDB Says both.

An early draft of the script had a male Ripley, making this one of at least three films where Sigourney Weaver played a character originally planned to be a man. The second is The TV Set and the third is Vantage Point.
All of the names of the main characters were changed by Walter Hill and David Giler during the revision of the original script by Dan O'Bannon and Ronald Shusett. The script by O'Bannon and Shusett also had a clause indicating that all of the characters are "unisex", meaning they could be cast with male or female actors. However, Shusett and O'Bannon never thought of casting Ripley as a female character.