Why would an owner decide to put in an ADS-B UAT transceiver versus an ADS-B ES transceiver? What are the advantages and disadvantages of both systems?
ADS-B UAT operates on 978 MHz and are available for aircraft that operate below 18,000 FT MSL. UAT transceivers can display aviation weather products (FIS-B) as well as traffic from ground radio stations (TIS-B). The airplane must be in a coverage area for TIS-B services.
TIS-B is intended to provide ADS-B equipped aircraft with a more complete traffic picture in situations where not all nearby aircraft are equipped with ADS-B Out. - AIM 4-5-8
ADS-B 1090ES operates on 1090 MHz and is required for aircraft operating above 18,000 FT MSL. The information sent over a Mode S transponder is greater and is called an "extended squitter". These aircraft will be able to display traffic directly from other ADS-B equipped aircraft and those with Mode S transponders without the need to rely on ground based systems. FIS-B is not available to these transceivers.
The picture below illustrates where 1090ES and UAT are allowed / required for aircraft after the ADS-B mandate in 2020.
If the aircraft will be flown above 18,000ft or in other countries with an ADS-B mandate, 1090ES is required. This applies to pretty much all "large" aircraft; they have no choice.
However, the US has an enormous light GA fleet with very vocal owners, and the FAA itself was concerned about 1090MHz congestion, so they added UAT (on 978MHz) as an alternative. It was assumed this would be simpler and less expensive than replacing every transponder in the entire GA fleet (which seems to be true), and they also added free weather (FIS-B) to encourage GA owners to choose UAT unless they actually need 1090ES, in case the price difference wasn't enough.
Note that both options offer TIS-B/ADS-R.