Having driven all of the above cars recently on test drives, here's what i think
Touareg - Good value, solid interior and exterior styling cues, but if you're considering higher end vehicles, this is a little out of place IMO. It depends on how crucial the name brand is to your decision.
Porsche - Pricey, and to be honest, built on the same platform as the Touareg. I've heard of some maintenence issues and the fact that you can get a fully loaded Touareg for less than a base Cayenne S (I'm assuming we're talking all V8 models here). Unless brand really matters to you, I wouldn't pay for the overpriced/re-badged VW, I'd just go with the original VW Touareg.
X5 - X5 4.4i is a solid vehicle. IMO it handles like a 330 sedan and the V8 power is really robust and can get you where you need to be without you doubting it's capability. The interior is up to par with the rest of BMW standards, even though it is assembled in North America. With options, it too can get pricey, but still not as pricey as the Cayenne S. I'd pick this over the Cayenne S, even though it's due for a redesign. It's a solid buy, and with BMW's full Maintenence program, you won't even have to pay for brake pads while you're under warranty. Something you won't get with other companies.
Range Rover Sport - I haven't had as much experience with these as the others, but IMO I wouldn't buy this car unless it was the supercharged version. The Sport exterior styling cues will set you apart from the already existing plethora of 4.6HSE Rovers already out there, but it's still got a bit of a bulky feel to it and doesn't handle itself as well on the road as the others. The Cayenne S and X5 both have a more 'sporty' feel to them, even though this is supposed to be the Range Rover Sport. I've heard of a few quality issues with this car, although it hasn't been out long enough to get any long-term reports.
I would urge you to consider the new Mercedes ML. Mercedes has really stepped it's game up with the new ML and the quality feels much higher than the original. The redesign did the M-Class justice, and the ML500 is quick. The styling redesign was needed and has been well accepted by many.
You really need to test drive all these cars and work out the pricing based on your budget. If It were me, I'd focus my sights on the ML500. If you're worried that the X5 redesign, than the ML is for you because it was JUST redesigned last year! If you get the optional AMG Package on the ML, you'll be one of a kind because it's more rare on the new ML since the base ML350 pricing is so reasonable, it's more common. The ML500 AMG Sport would be rare, and fun. The Cayenne is overpriced IMO. My conclusion: Go with what you know; BMW won't let you down, but since you need a car now and the new X5 isn't out (if you're worried about redesign), the M-Class may fill that void.
the one on the right is the ML AMG Sport Package
the optional aero kit for the X5. Those are 4.8 wheels, but the wheels offered on the 4.4 are good too.