Between the two, I'd pick the X5.
I've talked to too many people with issues on their early ML's, and I just hate the way they look inside and out. I think they're kinda cheap.
I remain impressed by the X5's looks, and if she's into how the SUV drives and she already has a BMW I'd stay to stick with it.
The early Touaregs had lots of issues. Most have now been solved, so I've been led to believe. Early issues to do with the tire pressure warning system going off all the time, wood cracking, electrical gremlins, etc. I would think the electrical issues would be shared by Cayenne. The V6 has as power boost in '05, and a new FSI V6 with 280hp will come this fall.
The SRX looks neat from the outside, but the golf-ball like dimples of the dash inside ruins that for me.
The RX 330 is a niiiiiiiiiiiice vehicle. Drove one for quite some time and I'm always impressed with it. Very capable, nice (but rather unexciting) to drive, perfectly sized for most people's needs, very smooth, has a distintive look to me, and they're ultra reliable compared to the European stuff. I still think it's the standout, as it offers so much in one complete package.
The CPO programs are interesting though. Pretty attractive deals to be had.