The antenna choice depends on the coverage needed. If you are installing these for user connectivity (not mesh) inside a building and on the ceiling, the 16 is probably the best choice (but that might not be true in all situations... this highly depends on your needs).
The e and h plane is explained pretty well here:
http://en.wikipedia.org/wiki/E-plane_and_H-plane
The 17 and 18 antennas are GENERALLY used for wall mounted coverage of warehouse or outdoor locations (for user connectivity) or for mesh APs.
The 16 will give you 180 degree coverage in front of the AP (useful for ceiling mounts), the 17 is a 120 degree antenna (reaches further, but wont cover too well side to side) and the 18 is a 60 degree coverage pattern, so it reaches further, but lacks "wide" coverage.