Tempered glass or film, which should I buy?

This is the first decision to get right, and it's less a matter of taste than most buying guides suggest. The shape of your screen largely decides it for you.

What tempered glass actually is

Tempered glass protectors aren't simply thin glass. A sheet of aluminosilicate glass is heated and then cooled rapidly at the surface, which puts the outer layer into compression and the core into tension. Because cracks can't easily start in a surface that's being squeezed, the result resists impact far better than untreated glass of the same thickness, and when it does fail, it fails all at once instead of splintering.

A typical protector is around 0.3mm thick and sits on a silicone adhesive layer that bonds by surface contact rather than glue. That's why a protector can be lifted and re-laid a few times: nothing is curing, it's adhering.

What film actually is

Film protectors are polyurethane or hydrogel, usually 0.15mm or thinner, applied with a pressure-sensitive adhesive. Better grades are mildly self-healing: the polymer has enough elasticity that light surface scratches close up over a day or two at room temperature.

Film flexes. That's its entire advantage, and it's a decisive one on any screen that isn't flat.

Why the screen shape decides

Flat glass laid over a curved surface makes contact in the middle and bridges at the edges. That gap is what causes the halo, the rainbow banding and the corner that eventually lifts. None of that is a manufacturing fault. It's geometry, and no amount of pressing will change it.

So: a flat screen takes glass. A curved-edge screen takes film, a case-friendly protector that deliberately stops short of the curve, or a UV liquid-adhesive kit that bonds glass to glass with no gap. A folding inner panel takes film only — tempered glass fitted across a crease will crack along it, usually within days, and can damage the display beneath.

Impact protection: not a close contest

On a flat screen, glass is the only one of the two that meaningfully protects against a drop. Film in front of a display does very little to stop the glass beneath it cracking, because film can't distribute a point load. It deforms locally instead of spreading the energy.

Film protects against scratches. If your concern is a key in a pocket, film is adequate. If your concern is the pavement, it isn't.

The differences people discover later

Film yellows. Polyurethane degrades under UV, and the change is in the material not on it, so nothing cleans it off. Cheap film can show a visible amber tint within months; UV-stabilised grades last considerably longer but are still consumables. Glass doesn't yellow at all.

Glass is far easier to fit. Modern glass protectors ship with an alignment tray that positions the protector for you and drops it straight; film usually has to be squeegeed out from the centre, and every trapped bubble shows. If you've botched three protectors in a row, moving to glass with a tray will probably fix that in one go.

Film is thinner and disappears against the phone. Glass adds a perceptible ledge at the edge unless the cut is very good, and on a phone with edge-swipe gestures a thick protector edge can interfere with them.

What to do

Flat screen, ordinary use: tempered glass, £10 to £20, from a brand that includes an alignment tray. Curved screen: hydrogel film for convenience, or a UV kit if appearance matters more than fitting difficulty. Folding inner panel: manufacturer film, replaced by the manufacturer when it wears. Outer cover screen on a folding phone: ordinary glass, no different from any other flat screen.