Choose antennas for RF (915 MHz)
An antenna with a gain of 9 dBi means it has a radiated power per square metre in its "best" (or optimum) direction that is 9 dB greater than an isotropic antenna in any direction. An isotropic antenna emits the same power density in all directions and although it sounds really useful is an impossible dream BUT useful as a concept. 

This means that the 9 dBi antenna MUST not be as good as an isotropic antenna in some directions and this could be a big problem to some applications.

On the other hand, a 2 dBi antenna is like a half wave dipole and it will produce a doughnut shaped power density that has 2 dB gain in X and Y directions: -



The down side is that it produces very little power (theoretically zero) in the Z direction. So, if you add all the power densities up and calculated an average you would find that the average power density produced is the same as an isotropic antenna.

What you gain in one respect you lose in another because a high gain antenna becomes very directional.