Thanks for the long explanation, that does clear things up for me, hopefully it helped others also.None of us really now all that much, some of us are just better than others at hiding it.
Short answer: If you're only really going to use it for low-frequency measurements, the basic model will be fine.
Long answer: The reason I have different options is that omni-directional mics (which are supposed to have the same response to a given sound wave no matter what direction it comes from) don't actually act omni-directional at higher frequencies. High-frequency wavelengths are relatively small, so when they come into contact with the microphone, they reflect off the mic in different directions so the response of the mic to those high-frequencies is heavily dependent on the orientation of the microphone to the sound wave.
The more-expensive mics I sell basically contain information to let a user know and account for the directional effects at higher frequencies. For example, if someone is using to use the microphone for high-frequency room-calibration purposes, it's useful to know the directional characteristics of the microphone so they could adjust their measurement methodology accordingly.
Low-frequency wavelengths are so long (~10ft at 100 Hz, ~20ft at 50 Hz) compared with the microphone that almost all mics act omnidirectional at those frequencies. So if you're only going to measure low-frequency stuff, it doesn't matter what direction you hold the microphone in and therefore the basic model (which doesn't include any of the high-freq directional information) should suffice.
Does this help?