Helix: 120-night trial, limited lifetime warranty. Casper: 100-night trial, 10-year warranty. Mattresses that are going to fail sag around years 8-12, so open-ended coverage genuinely covers the risk period in a way a 10-year clock doesn't.
The bigger difference is that only one of these is still made. Casper has discontinued the Original Hybrid: casper.com now redirects it to the Casper Dream Hybrid, and GoodBed lists the Original among Casper's discontinued models. What remains is Amazon stock under ASIN B085H9FBBP, flagged as limited quantity. Helix Midnight Luxe is built to order in the US and ships in a few business days.
For many buyers that outweighs the price gap, which is now wide: the Casper costs about half as much. A 10-year warranty on a discontinued line has no current-production unit to exchange into, and if the bed doesn't suit you there's no Casper-direct replacement — you'd be moving to the Dream Hybrid, which is a different mattress at a different height and price.