I ordered a Macan a while ago, after I talked about it with a guy who parked behind me in the building garage, he bought a Cayenne. By the way, on my left side is an Aston Martin.
I am amazed by his black Cayenne, and decided that Cayenne is a more aesthetically designed car from all angles. Therefore I cancelled my Macan order a few days ago. And I will buy a Cayenne instead. Beside, Cayenne isn't that more expensive than a Macan in the first place. You won't save a lot of money buying a Macan anyway.
So is there any real advantage of Macan beside the smaller profile?
First off, you probably should just as well post a similar note on the Cayenne boards to get their opinions. Coming here, to a board of anxious prospective Macan owners, you mind as well ask why bother with a Macan over an X3/X4?
The Cayenne (at least in it's 958 incarnation, which is all I know) is a fantastic vehicle. I ordered mine in Sept '10 and honestly wasn't in the market for another vehicle. The handling, agility, fit, and finish, outside and in, are all outstanding on the Cayenne. I have the V6 version, so it's not fast by any means, but I've never been as happy with a vehicle.
Aesthetics are such a personal preference, but I'm always baffled by those who seem to have disdain for either the Cayenne/Macan and really like the other. Although there are many differences, there is no mistaking the bloodlines, you can tell they are closely related. Again, personal preference. That said, I'm excited for Macan because of the slightly smaller profile, PDK, and (from all accounts) sportier feel. It also helps that a Turbo variant of the Macan is a good $40k less than a CTT (in the US at least). You get a lot more for the money with the Macan, and it's really a different class of vehicle IMO- compact vs full size SUV. I'm ready for something a little smaller and faster, while still maintaining the practical aspects of what I have in the Cayenne.
So, going back to your question, it really depends on what you want and what you're looking for- they are different vehicles and fulfill different things for different people. Will there be some cannibalization? Sure, but there is def room for both of them in the line up. The Cayenne is a great vehicle, and I'm sure the Macan will be as well. The best thing is, they are both from Porsche, and as cliche as it sounds, there is no substitute.