Radonaway has the least expensive sniffer, and very functional- can get saturated with multiple high readings but great device for the money.

These are used for mitigation only. There have been inspectors who've used these during an inspection to 'perform a radon test' and that is NOT ok

Thanks Robert and Karen. I was able to get some information from Radonaway. Yes, this is for troubleshooting mitigation difficulties and finding sources of radon, as opposed to monitoring for levels. Thank you for your feedback!

I have a W. Johnson sniffer, which I understand is very highly thought of. They are no longer made, but sometimes become available on ebay or from retiring radon pros. I bought mine from used form another inspector, and have heard that the manufacturer maintains a wiring list. My sniffer CAN be used to perform a professional radon test, Karen, as it has capability of measuring 48-hr intervals, as well as very short sniffs of few seconds.
The RadonAway version is simpler, and also functions as a manometer.
There are also the RadElec RAD7 (more of a lab instrument), and the CT007R and VS472 from radon sniffer.com, which I know nothing about.

We have used the Durridge (formerly Niton) RAD7 several times for paired (indoor air and sub-slab soil gas) radon testing. It works well for field investigations, provides timely results in the "sniff" mode, and can be rented or purchased from the manufacturer. It's also an approved continuous radon monitor:

http://aarst-nrpp.com/wp/approved-devices/

Here's some more information on it:

https://durridge.com/products_rad7.shtml

We have used a Pylon AB5 radiation monitor. It's a research level instrument and very versatile. It can be used for sniffing, short-term and long-term radon measurements. I have seen several recently listed on Ebay for $4-500. There may be additional costs for scintillation cells which can be purchased from http://radoncorp.com/testing/pylon-monitors.php in the United States.