IMHO, I believe the only thing those cc types are good for is in the event of an nuclear detonation, or catastrophic accident. In other words, you'll probably never have use for it. It only registers very high radiation. I think you can't re-charge them either. Once they show anything, (like possibly a year long accumulation of background), you can't put them back to zero.
You can get a dosimeter pen and charger on E-Bay for less than that, that registers much lower radiation. And those can be reset.
As to the key chain ones. The smaller the Geiger tube, the less sensitive it is.
Less particles of radiation can be "caught". I think they are usually sensitive to 75 mR/h and over as a general rule. Again, okay for high sources, not too good for the more common lower ones.
