14.5 is the "old" standard. It was developed from an even earlier "composite" standard which combined cycloidal and involute gear profiles. I believe this is still used in watches and clocks. A variant of composite is used for molded plastic gears.

20 degree gears are stronger, suffer less from undercutting for small numbers of teeth.
Supposedly they increase forces tending to push gears apart, but I have never seen a case where this is a problem.

Worms sometimes use even greater angles for more strength.

20 degree gears should be chosen in 99.9% of cases.

Some references: Guillet's Kinematics of Machines, Machinery's Handbook. Marks Standard Handbook for Mechanical Engineers.

The standard specifying 14.5 pressure angle gears was discontinued in the early 1970's. As there are thousands of inch pitch gears with this pressure angle in use, numerous stock gear suppliers still manufacture to the old standard.

20 degree gears are the standard throughout Europe and Asia and they have been the standard in the US with the publication of AGMA specfication 2001-B88.

The advantage of 14.5 degrees is a smoother mesh, resulting in lower noise. The disadvantages are less strength and a higher threshold for undercutting.

While 20 degrees is the current standard, there are many applications where a 25 degree pressure angle is used. These are applications where a strong gear is required but the number of teeth needs to be small and there are no restrictions on noise.

Historical note:  The 14.5 degree pressure angle was first chosen because its sine is very close to 0.25, which simplified calculations.