I would wait until the 08 Viper comes out to make a judgement. I do see, however, that any GT's that are sold on the open market have a grand total of 1k miles or so. These cars are just not driven. If you want to have a "collectable", a majority of the cars that I can see are simply not driven or driven very very sparingly, thus if you drive your car, you may hurt the overall collectability.
2000 cars is a lot of units, in my opinion. Take an even rarer car, the Jaguar XJ200. Very few of these cars were ever made. Costs were in the 600k range. Now, you can pick them up for the price of the GT. Super rare, super high speed performance....and still no appreciation in 15 years.
And forgive the horrible comparison (but I needed a more modern American counterpart car), but look at the Turbo Trans Am in 1989, or Formula Firehawk in 1992. At its time, these cars were the GT of their day. Limited numbers. Did not really matter in the big scheme of things.
My suggestion is that you give both a try and buy the one that best suits you and forget about appreciation for the time being. I just don't see the prices of the GT flying off the radar screen within the next 3 to 5 years if you want to change your mind and you originally decide that the 08 is the car for you. That being said, however, I must admit that the GT is a fine piece of engineering and some colors are extremely hard to find. Black or dark blue, without stripes or with the single stripe on the side is almost impossible to come across. If you want that combination that is almost impossible to find, then I say pull the trigger now. You may never see another one.