Forgot to mention the the Fieldpiece humidity head does also do temperature.
That unit from Radio Shack looks ok. I've bought a couple different table models before and have been asked many times to confirm customers tabletop Humidity monitors and I have found some to be right on the money and some total junk. It's kind of hit or miss there. If you have a buddy in the HVAC field ask him to confirm your tabletop model with his equipment for you. Or you could ask the heater/A/c guy to if you have one in your home for a furnace service ot A/C estimate etc.
There doesn't seem to be a correlation between price and accuracy. I have seen the 8 dollar units right on the money, and the 30 dollar ones similar to the RS model linked to, to be way off. So as I said, it's hit or miss there.