A SPL Meter's primary purpose is to measure loudness. (like setting level in a multi-channel system)
A Calibration Mic's primary purpose is to measure frequency response. (like adjusting cross-over settings)
That's not to say that they can't do the other task, it just what they are optimized for.
Which are you planning to do.
To be honest I dont know! haha. I just notice that a lot of people are utilizing then when setting up their systems. I just did a quick Google search and found that some arent that expensive, so I figured maybe I should get one? Maybe it helps but initially I will be setting up a 2.1 system, and later expending to a multi-channel. I am just trying to learn.
You can get an spl meter app for an iphone that does a pretty decent job, if you just want something basic. Otherwise, the standard radio shack meter runs about $35 and does a fine job.