The model S is an all electric vehicle, not hybrid like the Volt. It pretty much is the closest car you can buy right now that can rival a gas car's range. You can buy it with either a 208 (per the EPA) mile range battery pack or 265 mile range pack. Acceleration of the car is rediculous. 5.9 second 0-60 if you have the small pack, 4.2 seconds if you have the 'performance' larger battery pack. The Volt is around 8.7 seconds (still no slouch). The Model S is 196" long versus the Volt 177". So, the Model S is significantly larger. You will also pay for all this.
The model S with the small pack starts at $62,400 US. However, the Volt isn't cheap either starting just under $40k.