Depending upon how well she ski's, 150's may be too long. If she's not a very athletic & just starting out my recommendation would be Demo, Demo, Demo. There are some great women's specific ski's out there.
I'd agree with Pwdrmama here. The SX9, despite being down the ladder from the SX11 of previous years is still a fairly stiff and aggressive ski. Unless your wife is a hard charger, there may be better female specific skis out there for her. If you're stuck on getting her the SX9, the 150 would be the way to go, unless she's into racing.