The Viper is a beast and handles better in every respect except low speed turning. The Viper is muscle car unlike any muscle car and is in a class of its own. There is not much out there that you could get for the money that would compare.

The Viper has the following advantages over the Xfire:

Better suspension handling
Better high speed cornering
Way better transmission
Way better styling
More reliable (a head gasket swap is the only problem on gen1's)
Way more attention getting, more stares and looks
Way more upgrade support and parts are actually cheaper
And of coarse the power, WAY more power even over an SRT6. (mid to low 12's stock)

Comparing the Viper to the 370Z, you will find that the 370Z has an easier shifter than the Viper and possible tighter in the turns. You have to be more careful in the Viper when corning because although it does very well, it's torque is nothing to mess around with. Never punch the throttle when entering a turn or in the middle of exiting a turn. Although a gen1 (1992 - 1995) Viper is rated at 400hp, simply upgrading the cat-back exhaust to a high flow system will net you about 450hp at the crank. I got 405hp to the wheels with just a catback and K&N filters so that's about 450hp at the crank.

Some of these comments may tick off Xfire owners but it's the real truth. The only thing better on the Xfire is the amazing turning radius. And as far as the reliability comment, I've had my Xfire in five times at the dealer within the first year of ownership and fix two things myself. They had to replace my Exhaust, fix a leaky valve cover, work on my headlights, fix my headliner, repaint a trim piece, and I've been having clutch squeal for a year now. The first gen Viper needs a Headgasket replacement at some point because the new one is an MLS steel gasket and the old one drips antifreeze over time, but that's it. I've had the Viper for over 3 years and I'm running a 150 shot of Nitrous in this 15 year old engine and it takes it like a beast. Think low 11's.