The AC Output from these sound level meters takes the raw signal from the microphone, amplifies it and applies some limited filtering. This output can be used for digital recording systems for example, or it can be sampled for post processing and further analysis. These meters meet the international sound level meter standards to Class 1 or Class 2, so the correct processing of the output signal will give a very accurate sound level reading.
The CR308 is the lower cost meter. The main limitation is that it comes with a fixed preamplifier (microphone stem), so it cannot take a microphone extension cable. If you are able to mount the sound level meter close to the monitoring location then this should not be an issue. Otherwise, the Optimus meter will be better as it has a removable preamplifier and microphone extension cables.
The CR512B Optimus sound level meter is also a data logger. Measurements can be downloaded over USB or it can even provide a live feed over RS232 (extra cable needed). Software is included to download measurements over USB and present reports.