What is the link distance? Longer distances normally mean lower frequencies. 23 and 24 GHz links are normally reserved for less than 5 miles. 18 GHz less than 10 miles 11 GHz and lower have been used to 60+ miles.
Throughput Any idea how much throughput you need? Tens of Mbps can be had with low cost unlicensed equipment. Hundreds of Mbps normally need full duplex licensed gear.
Full Duplex VS Half Duplex This is a latency issue. Licensed radios are almost all licensed full duplex with latencies in the single digit mS or even in the sub millisecond range. Unlicensed radios are almost all time division duplex (TDD) with latencies from 5 to 50 mS with WiMax being among the worst in this respect.
What's the terrain & trees in between (AKA is it LOS, or not)? Anything above 5.2 GHz needs a pure LOS path with little or no fresnel zone encroachment. There are 3.65GHz and lower MIMO radios that work with near LOS to varying degrees.
Is power consumption an issue? On mountain top repeaters that need to be solar powered, many full duplex radios are impractical to power from a cost standpoint.
What frequencies are available Unlicensed: 900 MHz(902-928) 2.4 GHz 5.2 GHz 5.4 GHz 5.8 GHz 24 GHz
Licensed (USA, FCC) 6 GHz 11 GHz 18 GHz 23 GHz Other higher frequencies are available)