Tuff call and what it depends on is what you do with it.
Here is what I know. I have had the opportunity to drive various Vipers.
They are hella fast, crazy monster power with handling that quenches adrenaline junkies. The down side... they are cramped, uncomfortable, hard to get in and out of (and I am not big) They bump along at slower speeds to almost a nauseating feel. They are NOT smooth for normal driving. The Viper will always bring more coin as they are a collector car.
Now for the challenger... the Challenger.
I have driven the six cylinder, the 5.7 auto, the 5.7 6 speed, the 6.1 auto, the 6.1 6 speed, and the 392 srt auto. They all are very comfortable, easy driving, and have great horsepower when asked for it. Even the six banger gets up and scoots ! By far the manual trans are a load more fun !!! I think they are a great comfortable, quick car, for the enthusiest. The problem... there are going to be more and more and more on the streets. The local "car shows" will have the die hard put his new Chally in it and I will walk by it. I will though... always... go check the Viper at the local car shows.
My Vote : Still the Viper.
-RPM