Each type of tower (outside) antenna, omni-directional and directional, has its advantages. Which is best depends entirely on your specific situation.
Please note that by FCC regulation, every weBoost cell phone signal booster comes as a kit with a tower-side antenna included. Only in rare circumstances should you need a different tower antenna than the one that came included in the kit you purchased.
Generally speaking, an omni-directional antenna works well in areas where the available cellular signal reflects off nearby objects. The reflecting objects may be naturally occurring - like trees or other foliage, hills or other terrain features. Or they may be man-made - like buildings in a downtown metro area.
The advantage of this antenna is its 360 degree beamwidth. It does not have to point in a certain direction to pick up a signal because it will bring in signals from every direction.
A wide-band directional antenna is the best solution when the available signal is weak. We recommend this antenna far more often than the omni-directional antenna because the directional antenna has double the gain (signal boost) of the omni-directional antenna. The directional antenna also works with the common frequency bands used for 4G LTE service.
A directional antenna also will help prevent feedback loops that can occur when the tower and device (inside) antennas pick up each other's signals. The directional antenna must be pointed in the specific direction of the cell tower, which requires a more precise installation. But this also serves to isolate the directional antenna signal and prevent feedback loops.
When using a directional antenna it's necessary to know the location of your carrier's tower. And if you need the antenna to collect signals from more than one tower, the towers must be located in approximately the same direction.
To learn more about omni-directional and directional antennas, watch this short video.