I like story driven action/adventure games that can be completed in 10 hours or less. The only times I have to play PS3 games are friday and saturday nights for anywhere between 2-4 hours each night. I have many other hobbies so I do not game every weekend.
My 2 favorite story driven action / adventure games that Ive played are: Uncharted and COD MW2. I was able to complete these games in reasonable times (1 or 2 weekends) I also played through God of War III, and while it was enjoyable, I preferred both Uncharted and COD MW2 (single player campaign) to it. Ive played my share of first person shooters over the years so I was surprised that I enjoyed MW2 as much as I did. I definitely do not want to buy any more "war" themed first person shooters, but I may be convinced in playing some other themed first person shooter (like sci-fi or alien themed).
Online play should NOT be taken into account when suggesting game titles to me. I want to find a short game with an incredible story to play over the holidays.
I dont care if the game you suggest is a new game, or an old PS3 game. I actually like picking up older titles because you can always find them cheap.
Have you considered MGS4? Very story driven and alot of action and or adventure. I love MGS4 but have been more into the God of War Collection as of late. God of War Collection is fantastic as it has the 2 first games.
I know on easy these games can be completed in not time at all.
But based on your "Sci-Fi/Alien" themed specification. I agree with the other poster. Dead Space might fit your needs but it is a horror game as well, so be warned.
besides uncharted 2 i would have to say Resident Evil 5 would be a good choice. I liked the story because I've been a fan since they first came out. It might take some time to get some good guns and beat it the first time through. The first time I beat it took me about 20 hours but with the right kind of guns i beat it the second time in under 5 hours