Both are convertible car seats — here are their published manufacturer specs side by side.
This compares published manufacturer specs — not safety or fit advice. The seat's manual, your vehicle's manual, NHTSA's tools and a certified CPST are the fit-and-safety authority; always check them before installing.
Spec
Britax Marathon ClickTight
Cybex Eternis S
Stage
Convertible
Convertible
Narrow (3-across)
–
–
Extended rear-facing
–
✓
FAA-approved
✓
✓
Lightweight / travel
–
–
How they differ
the Cybex Eternis S lists extended rear-facing (a higher rear-facing weight limit, ~40-50 lb); the Britax Marathon ClickTight doesn't
Published limits
Mode
Britax Marathon ClickTight
Cybex Eternis S
Rear-facing
5–40 lb
4–50 lb · 17–48 in
Forward-facing harness
22–65 lb
22–65 lb · 28–49 in
Booster (with back)
—
40–120 lb · 44–57 in · 3+ yrs
Useful life
10 yrs from manufacture
—
Britax Marathon ClickTight: Rear-facing: 5–40 lb and 49″ or less. Forward-facing: 22–65 lb and 49″ or less (printed limits, same height cap both modes).
Cybex Eternis S: All-in-one (rear-facing → forward-facing harness → high-back booster; no backless mode found in manual). Forward-facing also requires at least 1 year old. Useful life is set by the printed "Expires On" date (no printed number of years in the manual).
That is the record of the brand — not a verdict on these models. A recall names specific models and manufacture-date ranges, and a seat missing from the record is not "recall-free": check nhtsa.gov/recalls with the model and date of manufacture on your seat's label. Source: NHTSA, retrieved 2026-09-05.
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FAQ
What's the difference between the Britax Marathon ClickTight and the Cybex Eternis S? The Britax Marathon ClickTight is FAA-approved for aircraft; the Cybex Eternis S is extended rear-facing, FAA-approved for aircraft. Both are convertible car seats.
Which is right for my car and child, the Britax Marathon ClickTight or the Cybex Eternis S? That depends on your specific vehicle and child — this page compares published manufacturer specs only, not fit or safety. Check the seat manual, your vehicle manual, NHTSA's car-seat finder and ideally a certified CPST before deciding.
Citing these specs? Go ahead — published manufacturer data, last verified 2026-06-11. Copy a ready-made reference: