I originally got a 996 Targa when I worked in Weissach. At the time, 997 production was only starting, and only department managers were allowed to order one. So, the 996 Targa was the cheapest 911 we could get.
The 996 Targa was a commercial failure, but it made about 25% of all N/A 911s on the Weissach parking lot when I was there. I sometimes suspect Porsche kept the Targa in its product range just for its engineers...

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I would not really compare a Targa with a Cab. Even with the windows and roof down, you still feel inside a coupé (albeit with a huge moonroof) because you see the sides of the roof (unlike in the pre-993 Targas).
Where the Targa really shines IMHO is... when the wheather is bad. The glass roof is amazing. The luminosity is incredible. At traffic lights, I could not help but look up at the raindrops beautifully flowing down the roof.
I'd rather compare the Targa with a regular Coupé. Unless you intend to track the car (in which case the reduced stiffness, the additional weight and the higher gravity center would matter), I see little reason not to prefer a Targa.
The glass roof is a huge plus. I'm not returning to a steel-roofed car again. Besides, the rear window opens like a hatchback. I used this all the time. It's very convenient.
I know it might sound odd to mention practicality in an exotic, but it does matter, especially since it was my only car. Indeed, the biggest advantage of a Porsche over a Ferrari is IMHO that the Porsche also is an everyday car. The 911 is a reasonable and fairly pratical sports car, and the Targa is the most reasonable and practical of all 911s.
On the negative side, I did not keep the car long enough to experience it myself, but I heard the roof might start creaking after a while. Whereas the 997 soft top has an excellent quality and is very durable.
(you might want to avoid the stone grey top, though, as it requires extra care to avoid black marks)
Jack of all trades, master of none.
Bottom line is, if you have another car and really want the full Cab experience, get a Cab. If it's your only car and you're not living in Florida or South California, get a Targa.
I had to return it when I moved to the US for a few months, but let me tell you I'm definitely getting a 997 Targa 4S next year.
Edit: the Targa will only arrive in 14 months. Whereas you could spend Summer '06 in a 911 if you stuck to your Cab order.