I have had the 8mm since I got if for my 16th birthday and I love it.
It has the vibe of a thick strand of pearls to me and goes with everything. At one point I also had the necklace and earrings too but I sold the necklace and lost an earring, oops. Anyway the bracelet is a classic and with the thickness just depends on the look you're going for. I like both but prefer the 8mm.