The Viper goes about its business a bit differently. It is more brute force than sophistication. No fancy dual clutch gearbox, no high revving V8, instead a big 8.4 liter V10 with 640 horsepower mated to a classic manual gearbox. The interior lacks the sophistication of the SLS AMG but it also lacks the sticker shock of the SLS Black Series undercutting it by almost $200k. You could have two Vipers and quite a bit of spare change for the price of the SLS AMG Black Series.
That said, these are performance cars so what are the numbers? Acceleration figures are in the favor of the SLS AMG Black Series. The Viper manages 0-60 in 3.5 seconds and the 1/4 mile in firstname.lastname@example.org. It's fast. The SLS AMG is faster with 0-60 in 3.2 seconds and the 1/4 mile coming in email@example.com. Impressive, unless one is about $ per performance which in which case these are two cars they are not cross shopping.
MotorTrend gets the weight wrong it's basically a wash. They quote the SLS Black Series at 37XX whereas Mercedes-Benz lists it at 3417. A mistake, but one MotorTrend should not be making.
What about around the track? The Viper's Laguna Seca laptime is a very nice 1:36.42. It is said to not be as planted as the SLS AMG which manages a 1:35.03. That's a pretty large difference and the Black Series better be quicker around the track for what Mercedes-Benz is charging. It's a phenomenal car. Both of these cars are.