Sand or gravel for a planted tank?

Both work. The difference is grain size, and grain size decides how roots, water and fish behave.

Sand (0.5–1.2 mm)

Clean look, good for carpeting plants and for fish that sift — corydoras, loaches. Roots get less oxygen, so root-feeding plants need root tabs.

Gravel (2–4 mm)

Water moves between the grains, roots spread easily and bacteria have more surface. Debris sinks in, so siphon it regularly.

Use both

Gravel at the back for stem plants and swords, a sand foreground or path in front. Separate them with stones so they don't mix.