Mattresses are such a personal preference thing. You gotta go to the store and try one first.
Just make sure to get a name brand that you can be fairly confident will still be in business 5 years from now.
I would be very fearful to buy a "bed in a box" from one of those startups, as there is a good chance that they won't be still be around by the time your mattress starts sagging a few years from now.