Difference betw. Unidirectional vs. Omnidirectional cell boosters

Omni-directional antennas and uni-directional antennas each have their own advantages. Which is best depends entirely on the specific situation.

Therefore, it depends on what you define as "better" because that could mean either easy to install and catches signals from all carrier towers when outside signal is good (omnidirectional exterior antenna cell phone signal booster), or it could mean catches signals from farther away but installation is a bit tedious with need to point it precisely towards the most relevant carrier's cell tower (unidirectional outside antenna cell phone signal booster).

FCC regulations require that every cell phone signal booster be sold as a kit with the tower side antenna included (synonyms: outside, tower, donor antenna). Only rarely would a different tower antenna be required.

An omnidirectional antenna generally works well in areas where the cellular signal available is good - reflected from nearby objects, either naturally or man. Omni-directional antennas have 360 degree beam width and do not have to be pointed in a certain direction to pick up signals.

Unidirectional antennas can be better solution when available signals are weak. This directional antenna is recommended far more often than an omni-directional antenna as it has double the gain. The drawback is installation time, and that it can usually target only a single carrier (unless other carrier towers are also in the same direction which is the case many, many times).

Another advantage of a directional antenna is that it 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.