This is education, not advice
The information here is written for the general reader and does not replace a personal medical consultation. It cannot tell you whether either option is appropriate for your chin, your health or your circumstances.
No outcome is guaranteed
No treatment can be guaranteed. An implant can move or become infected and may need to be removed; filler is absorbed at a rate that varies; and a result is judged after the swelling has resolved rather than in the first days.
Neither option makes a chin smaller
Both options add. A chin that is too prominent for the rest of the profile is a different problem, addressed by reducing bone or by changing the proportions around it, and adding either material would make it worse.
A bite or airway problem is not treated here
A small lower jaw, a bite problem or an airway problem can present as a chin concern and is assessed as a jaw concern. Treating it with volume or with an implant leaves the cause where it was, which is why the assessment precedes any plan.