I can't give you an honest "PS3 vs. 360" answer, because I just don't know the 360 well enough.
However, if she ends up getting a PS3, I think these games should be her style:
- Uncharted
- Ratchet & Clank (they're cinematic and don't take themselves too seriously, great games)
- Flower (this is an absolute must)
- The Last Guardian (based on the ICO pedigree alone)
- Heavy Rain (It can't be said enough 
)
- Blue Toad Murder Files (if she like 'whodunnit' TV shows, this should be a good fit. You should read up on it before recommending it though, I have no idea if it actually fits)
- LittleBigPlanet (I haven't met anyone who hasn't liked it so far)
- L.A. Noire (based on what we have heard so far, this might be right down her alley)
- Heavenly Sword (despite its shortcomings, it's not a bad game, and it is very cinematic and the story is very well told. In that respect, it's up there with Uncharted if you ask me. And the asking price is now pretty in line with the value you get from the singleplayer too.)
- Prince of Persia (from last year, should be good for having something that isn't overwhelming to play)
- Valkyria Chronicles (if she's into strategy. It's a good and well told story, with good english dubs)
- Braid (haven't played it, but it should fit her from what I have heard. Disregard if you think otherwise)
That's all I can think of at the moment.
EDIT:
- Pirates of the Caribbean DLC for LBP (just in case she likes Pirates of the Caribbean )
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