The structural difference
Astrology is calculated. Given a date, time, and place, the chart is determined: two astrologers working from the same birth data produce the same chart, and the interpretation is where they differ. Tarot is randomised. The same question asked twice produces different cards, and the draw is the variable rather than the input.
This single difference drives almost everything else about how the two feel to use. A birth chart is a fixed object you return to for years. A spread is a moment, and it does not persist.
What each is actually good for
Astrology's natal chart is a description of a person rather than a situation, which makes it well suited to questions about temperament, recurring patterns, and long arcs. It has a vocabulary for "this is how you tend to handle conflict" that tarot does not really possess.
Tarot works at the scale of a situation and a moment: this decision, this week, this relationship as it stands now. It is far more responsive to a specific question, and considerably worse at saying anything durable about who you are. Readers who use both generally use them for exactly these different jobs rather than as alternatives.
Where they overlap
There is a genuine historical connection. The Golden Dawn and the occult revival of the late nineteenth century assigned astrological correspondences to tarot cards: the Emperor to Aries, the Chariot to Cancer, and so on through the Majors, with the Minor Arcana mapped to decans. Those correspondences appear in a great deal of tarot writing.
Whether to use them is a genuine choice. They add a layer of structure and they also add a system to learn that has nothing to do with reading the images, and plenty of excellent readers ignore them entirely. If you already know astrology, they are a bridge; if you do not, they are a second subject.
Which is easier to learn
Tarot, by a considerable margin, and mostly because you can start badly on day one. Pull a card, describe the picture, guess, look it up, that loop produces progress immediately. Astrology requires understanding planets, signs, houses, and aspects before a chart says much of anything, which is weeks of study before the first satisfying result.
The trade is that tarot's ceiling is reached through practice and reflection rather than through accumulating technical knowledge, which some people find frustrating in the opposite direction.
The claim each makes
Both are used predictively and both are used reflectively, and the honest version of each is the reflective one. Astrology read as fate and tarot read as fortune-telling make the same category of claim about knowable futures, and it fails for the same reasons in both.
Read as structured frames for thinking about yourself and your circumstances, they are two different instruments with different resolutions, one for the person, one for the moment.