Go with the 08 for many reasons, better tranny, better electronics, better tires, more n/a power, etc. There are guys out there that have run stock 08's against Paxtons Vipers, and surprisingly the cars are very close in performance. There was a guy at Saratoga this past Saturday w/ a 1000 hp Twin Turbo Viper, and a new 08. He is selling his Twin Turbo and keeping the 08. Again, he said the 08 performs close to the TT due to traction problems of the TT and the fact that the TT acts "squirrelly" at times. Now, understand that if you line up a 750 hp Paxton or 1000 hp TT against a stock 08 and run the cars 0-190 mph, the Paxton and the TT would beat the 08, but from 0-140 it would be close between the Paxton and the 08. And the 08 would beat the TT 0-100 mph unless the TT had major tire and suspension upgrades. If you go on to add Stop Tech rotors, headers, cam, free flow exhaust, K&N's, lighter wheels and tires to the 08 you decrease rotating mass and throw on some 3:33's, you will have a monster that will beat both FI cars, the ZR1, and practically everything shy of a Bugatti all the way to 160 mph in my view.
You can add headers and cam alaone to the 08 and have almost 700 hp (over 600 rwhp) and have a car that is lighter and quicker than most Paxtons out there. And much quicker than a slightly quicker than a TT to 140 mph (again unless the TT has major tire and suspension upgrades like Motons). The 08 will be a better track car all around due to running cooler than the FI cars and not having the extra 90 lbs in the nose of the car. As far as th e Roe goes, I would prefer the power coming on later in the power band like a Paxton than having all that torque at 2000 rpm like the Roe. SRT's are torquey enough as is. Having said that, there are no Roe SRT's out there yet to really get an idea of how the car will run.
Also, buyin g the 08 helps car the Viper brand strong. Get the 08.

And order it from Tators Dodge!