It's not a huge difference, I think - I'll use any bed for my teens and adults that has an 8 or better. However, all my Sims get the 'seldom weary' trait as soon as they have enough points (usually when the kids complete three of the four aspirations). I put all my teens and adults to bed at 1:45 and whether they have an 8 or a 10 bed, they are ready to get up at 5:00. (I like to have them sleep a bit - I don't like the 'never weary' trait - but I'd rather have them awake and doing things. Watching them sleep is rather boring.)
Something I miss from Sims 3 - the dreams kids and teens could have at night. I'd love to see that come back.