There are differences in speed:
- mhz, which determines the clock speed (1600, 1866, 2133, etc.)
- cas latency or CL, which determines how many instructions can be done per clock cycle
Some memory can be more expensive because of heatspreaders being mounted on them, allowing cooler operation or (more room for) overclocking.
Since you are getting an additional one next to your current ram, you should get one that matches your current ram's speed or faster. Your computer will always set the ram speed of all ram at the same speed, and will set it at the speed of the slowest ram.