Both have their niche.
Base reflectivity depicts the location and intensity and location of precipitation reaching the ground. For practical purposes, base reflectivity is ideal for identifying where the snow, rain, or hail is actually reaching the ground and at what intensity. It also can provide a purposeful overview of storm cycles from a precipitation point of view as well (during convective activity). Ultimately, the further you look from the radar site, the higher the angles are and precipitation may be depicted as well. If you were landing at SLC with convective activity reported in the area and you were within the terminal range (< 30), I would use base reflectivity.
Composite reflectivity is best used for analyzing the moisture in the air (prior to falling to the ground) as it's a compilation of the radar sweeps. It is best used to analyze long distances or en route weather since it will show the amount of moisture or instability the air contains. Think of composite reflectivity as a forecast per se. It simply depicts the *potential* precipitation that *may* occur.
In essence, one shows what is capable of happening (composite) and one shows what is happening (base). I always taught and used one as more of an en route product and one as a terminal product.