The choice between circular polarization antennas and linear polarization antennas can make a significant difference in an RFID system.
Linear polarization occurs when electromagnetic waves broadcast on a single plane (either vertical or horizontal). Linear polarized antennas must have a known RFID tag orientation and the RFID tag must be fixed upon the same plane as the antenna in order to get a consistent read.
Due to the concentrated emission, linear polarized antennas typically have greater read range than circular polarized antennas of the same gain.
Compared to linear polarized antennas of the same gain, circular polarized antennas will have a shorter read range because they lose about 3 dB splitting their power across two separate planes.
When deciding which type of antenna to purchase for your RFID system, understanding the how these antennas work as well as how the RFID tags should be oriented with respect to the antennas is key.
If all the tags you need to read will be on the same plane and aligned with the plane of the antenna, then you should consider a linear polarized antenna. If your tag orientation is not something that will be reliable or consistent, then you will likely need to use a circular polarized antenna.