Try reading the tests on this page:
http://www.guysoflidar.com/
They do comparison tests every year or two and they provide quite a bit of detail.
Personally, I've been using a V1 since 1998. It saved my ass countless times. I sent it in to be upgraded to the latest version for $185 a year or two ago. Its much cheaper to upgrade it every so often than it is to buy a new top of the line detector.