Tooki or Dension for full featured integration. We've carried these products for nearly six years. Depends how much you want to spend, how well you want it integrated, and any other features you may wish to have.
The Dension (we have been the primary distributor on this for the first five years of product life) is the defacto standard for ipod and bluetooth integration for Porsche ('03 through '08). Not to say other products won't work. Just that we have the largest installed base of Dension product for Porsche, and it offers iPod, USB, Aux and Bluetooth, for a very reasonable price.
Dension worked directly with Porsche for the first year of development of the product, and we worked directly with Dension for that year providing info for them on the US Porsche market.
I have no knowledge of the iTronic product but it appears to be similar to the other major players (mObridge, Tooki, Dension). And if it does what it claims, it appears to be a very nice product as well.
Budget may play a role too - for our current discounted sale price of just north of $500 customers get iPod, Bluetooth, Aux and USB. For many of the other products, pricing for this package would be $800 or more.
Again, not suggesting one is better than another - you make your own judgement.
Best of luck with your decision either way. If I can be of any help, feel free to email anytime: email@example.com