A licensed Microwave system is more desirable where long term reliability is an issue. Here you are issued your own frequency by the FCC and theoretically you should (and I have never seen a case otherwise) have zero problems from the radio emissions of others. The downside of this is you may have to wait for a period of time while the frequency search is performed, preliminary notices are filed and the FCC license is issued. Additionally, there are fees involved.

The Spread Spectrum (unlicensed) is available in  900Mhz, 2.4Ghz, 3.5Ghz, 5.8Ghz and now 24Ghz! Spread Spectrum is suitable where some interference may be acceptable (typically voice only circuits), the structures are very close together or in a very remote area. Experience has shown us that remoteness and close proximity of antennas may not always overcome all interference, however we have had excellent success with these radios. The upside of the Spread Spectrum system is that no license is required so there are no fees and you may operate immediately!

My personal suggestion is to use a licensed system wherever reliability is essential.

Chris Herrington FCC License #P1-11-58915