Which hygrometer is more reliable?

You'd have to look up the specs from the manufacturer. +/- 5% is typical. I wouldn't buy anything with a higher +/- error%. Some of the less expensive ones are +/- 3%, too. so, of the typical ones you find, the 3% error are the better ones to get. These cheap devices work with a 'filament' or 'hair-like thing' that contracts in a consistent way due to RH levels. It's possible this just gets fucked up and it no longer works, or maybe you can re-callibrate it, if the device allows such a thing. Maybe hold the button down for 10s and that enters callibration mode? Need the manual to be sure. May or may not have the ability to do that, but some do. The good thing is you can probably tell which one is accurate without a 3rd probe, if it's off by 20%.. common sense will tell you which probe is fucked up. humidity will be stronger at the bottom. Someone is going to throw a fit i contradicted them, but it is not personal. There are two reason.. for one, humidity in the air is denser and settles toward the bottom.. it is also cooler near the floor or lower levels, which if absolute humidity is the same, then RH rises as the temps cool, because it is "relative" to temperature. Two reasons why you should expect higher humidty at lower elevation in any closed system. See ultraviolet.. nothing personal. If you get upset about being wrong, look inward and stop blaming external sources. Learn something instead of being obstinate and angry about it.