Well the main difference in Western Vedic astrology is the tropical/sidereal difference as to the world Western astrology is most famously referred to as Tropical astrology.
The problem in disproving astrology is the fact that when telescopes were discovered the position of the planets were different as to what the astrologers referred. The astrologers did not explain the facts clearly. Many western astrologers believed the earth was at the center of the universe.
Astronomy proved this fact wrong.
The sidereal concept of time is not based on the solar time and that's why the concept of free will may not be very evident in sidereal or vedic astrology as the SUN is the closest celestial star on Earth and this energy has more influence on way things occur on Earth.
The year is 365.24... days based on solar time. Millions of years ago this number was different. So astrology will be different in that case.
A lot has to be clear before people accept facts in astrology.
Actually, the main difference between Tropical and Sidereal astrology lies in how the zodiac is demarcated. I am just beginning to study this. There alot of mathematical fine points that I am studying. But this is my understanding of the basics in a very small nutshell.
The Tropical Zodiac begins at the Vernal Equinox, also known as the Aries point, the first day of spring. This is the point where the ecliptic intersects the equator. It is a point in
time. The mathematically demarcated signs of this zodiac do not match up with the constellations in the sky.
The Sidereal Zodiac begins at a point in
space. It is measured from Spica (Virgo 30), or alternately from Aldeberan and Antares (Taurus 15-Scorpio 15). The signs of this zodiac are pegged more closely to the constellations in the sky.
There are many, many more fine points to each system. And there are valid arguments in favor of each. I think that both are valid, and each one is relevant and symbolizes a different plane of consciousness.
There are so many competing and seemingly conflicting theories in the world of science. I use the example of Euclidean vs. Non-Euclidean geometries. These are two very different theories, yet they are both accepted without de-validating the science of geometry. Maybe someday both Tropical and Vedic systems will both be accepted as different facets of the spiritual science of astrology.