It comes down to how much healthy tooth is left
A filling repairs a small to moderate area of damage, usually from decay, while leaving most of the natural tooth structure intact. A crown covers the entire visible tooth, which is needed when too much structure is missing or weakened for a filling to hold up reliably under normal biting force.
When a filling is enough
Small to medium cavities, caught reasonably early, are usually well handled by a filling — the damaged area is cleaned out and filled, and the rest of the tooth carries on as normal.
When a filling isn't enough
If decay is extensive, if a tooth has cracked, or if there isn't enough healthy structure left to support a filling securely, a crown provides full coverage and support instead. A tooth that's had root canal treatment is also usually crowned afterward, since it tends to become more brittle over time.
Why placing a filling where a crown is needed doesn't hold up
A filling that's too large relative to the remaining tooth structure is more likely to crack or dislodge under normal chewing force — this is often exactly how a tooth ends up needing a crown or root canal treatment later, when a crown would have been the more durable choice from the start.
The honest answer from a good dentist
A dentist looking at an X-ray and the tooth directly can tell you which of the two your specific tooth needs — it isn't a matter of preference once the extent of damage is clear.