Is PS3 with Move an alternative to Wii?
Solution 1:
There will be fewer games that support Move at the moment because it's a relatively new device. If you look today you might not find the games you want on the PS3, but as time passes more games will be released that make use of Move.
Digital Trends has an article that compares the Move to a Wiimote. The basic difference is:
While the accelerometers inside the controller act much as a Wii does, the balls provide what Sony calls a zero point – an absolute location in physical space for the system to peg all the other data to.
While the Wiimote uses its camera to locate the IR emitters in the sensor bar placed above or below the screen, the PS3 uses a camera to locate the glowing ball on the Move. I've never used the Move so I can't tell you whether it's better or worse than the Wii.
Solution 2:
Apart the technical part, you also have to keep in mind that Wii is targeted for casual gamers, while the PS3 is more focused on hardcore gamers.
While you will find now or later sports games compatible with the move, it may well be that they will be much harder to approach than their Wii equivalent. You will have much more simulation games in sports than casual games.
My wife also likes playing the Wii as the system is really simple with movement and around 4 buttons to use, while she dislikes the PS3 (we don't have the Move). It may be different for your girlfriend but keep it in mind and check first the games she could be interested in to see what they propose on the PS3.
Solution 3:
If you are looking for a console that you will both enjoy, I would recommend the PS3. The Wii's hardware is rather limited when it comes to 'proper' games. With the PS3 you are getting a much more powerful console (not to mention a blu-ray player).
The Move system is extremely fast and accurate; in my opinion more so than the Wii.
Solution 4:
Wii is definitely better for casual gamers who like Wii Sports-like games.
However, it's not nearly as good a choice for serious gamers. The PS3 Move is really brand new, but already has several Wii Sports-like titles out, and as a bonus will in the future probably be supported by other more serious games.
If it was just for her use, I'd recommend Wii, because it's easier to use, and more well-established, but since you wish to use it for serious gaming as well, and also because Move has more potential, I'd recommend the PS3 Move.