Either of those systems are perfectly fine for a 12 year old girl provided the games that accompany it into your home are not in contrast to how you parent your child.
The Playstation 2 is still in production and has a TON of games ranging from E for everybody to M for mature. In addition, the Playstation 2 can be found for a very cheap price (saw one for under $100 the other day) and usually comes with some bonus such as an extra controller or a free game.
The Playstation 3 is also a solid choice for gaming. However, there are not nearly as many games for this system as the Playstation 2. It also costs more to intially purchase, and the games are more expensive. On the plus side, the Playstation 3 also doubles as a Blu-Ray player. This will allow you to play those type of DVD's if you keep the system in the living room for example. This may make it a worthwhile investment if you are looking for two things in one.
As for games, we can visit other genres of video games such as fighting games, platform games, and adventure games. I will give a small list of games for each system to start based on what I read.
Tekken Tag Tournament
Jak and Daxter series
Ratchet and Clank series
Little Big Planet (1 or 2)
Motor Storm series
Marvel vs. Capcom 3
FIFA soccer 11
The Bigs 2 (baseball with an arcade flare)
Rango: The Video Game
Finally, if you do purchase a Playstation 3, you can go online to the PSN network and download a variety of games for usually small prices. Amusing games may include Castle Crashers, or Everyday Shooter. The amount of games are astounding, so it may be better than purchasing a new game from the store.
Hope this helped, and good luck!