Should I get an air sprung or a coil sprung rear shock FS mountain bike?
You're mixing up two different concepts.

The damper, which is what slows things down.
The spring, which is what holds the weight and absorbs shocks.

The damper is usually hydraulic, oil being forced through a small hole.
The spring is either a coil or pressurised air.

Without a damper, you'll have a very bouncy ride and without a spring, you'll have a very stiff ride. The 2 work together to provide a nice ride.

Also, those 2 bikes target different styles of riding.

The Slash is an Enduro bike and the Voltage a Freeride bike.