It seems to be a pretty big difference between the lowest end and the beds with 10 energy, like a savings of a few hours less sleep per day. I tend to send my sims to bed by midnight and have them wake up around 6 am regardless of actual need (and use a mod so they don't wake up even if energy is full), so for me any bed higher than around 4 is good enough.
I've never actually timed it or anything, but my recollection is that a bed with energy of 4 results in sims that need around 8 hours of sleep in a sim day, while a bed with 10 it is somewhere between 3 and 4.