Directional antenna vs. an Omni-directional antenna: Which is better?
Omni-directional and directional antennas each have their own advantages. Which is best depends entirely on the specific situation.
FCC regulations require that every cell phone signal booster be sold as a kit with the tower side antenna included. Only rarely would a different tower antenna be required.
An omni-directional antenna generally works well in areas where the cellular signal available is reflected from nearby objects, either naturally or man. This antenna has a 360 degree beam width and does not have to be pointed in a certain direction to pick up signals.
Wide-band directional antennas is the best solution when available signals are weak. This antenna is recommended far more often than an omni-directional antenna as it has double the gain and it also works with the frequency bands commonly used for 4G LTE.
A directional antenna will also prevent feedback loops between antennas as the directional antenna is pointed in the direction of the cell tower, thus isolating its signal and preventing feedback loops.
Above is a basic response to the question posed in the title. For a more detailed response with video illustration, please see this blog post: Donor Omni-Directional Omni Antenna vs Uni-Directional Yagi or Log Periodic Antenna.