Are you asking about international chains? In that case they are (eventually) equivalent as they match each other quite closely (though it may take a few years for all hotels to get upgraded). I like the Westin "heavenly" bed but lots of hotels have matched it.

The very best beds are not in the chain hotels. They are in one of 3 places: ultra-luxury boutique hotels (with their 1000 TC Frette sheets, etc.), some B&Bs particularly in resort destinations, and the most surprising: "cheap" motel equivalents in Finland (cheap in quotes as while it is a Motel 6 concept the price is often USD 100 range and meticulously clean if spartan). The most comfortable bed I've ever experienced was about 50 miles from HEL. Truly heavenly.