Answerman
I think the answer may be right in front of you.
I also had some pretty fast cars (Porsches, Vettes, XK8, Maserati Spyder GT) and I have found that the XLR gets more attention than these cars (the Maserati did get almost as many looks from the few people that even knew what it was). I haven't had an SL, but there are just so many that seeing one on the road just isn't distinctive. The Viper is really nice (a very close friend of mine has one) and it is fun to drive but it is not easy to drive, nor is it comfortable. Further, the Viper hops around bumpy corners and is really just a "fair weather" car.
I can also tell you that the comments about the insurance need to be considered carefully. It will be VERY expensive IF you can find an underwriter willing to write the policy. If I were an underwriter, I just wouldn't want the risk with an 18 year old driving a nuclear missile.
The XLR is a very comfortable ride, it has reasonable power and you can use it virtually everyday (and on long trips -- which would be HELL with the Viper). The answer: WAIT FOR THE XLR-V and get great power wrapped in a comfortable package. The XLR-V is it!!! I know it is very difficult for an 18 year old to be patient (especially with $100K burning a hole in his pocket), but try to be patient --- it will be well worth the wait.
By the way, if all you have left is $100,000 to spend on a car, you better think about your priorities at your age UNLESS you already banked $2,000,000+ from the inheritance and have it invested safely. From the mouth of a CPA!!