High-back booster or backless booster — which is safer?

High-trust testing publishers such as consumerreports.org explicitly note that several popular two-stage boosters (including the Britax Skyline and Highpoint) deliver materially better belt fit in high-back mode than as backless. High-back also adds head and side support for sleeping kids. Backless is generally treated as a travel/secondary option once a child reliably maintains good belt position.