Though size 6 has more boob coverage, for some reason I think it looks a little weird. Kind of like I'm wearing a bra that is a size too big for me. Size 4 definitely gives more cleavage.
I love the color of this tank, but I can't help but think how even more beautiful it would look in lullure! Lullure is slicker and has a subtle sheen to it, which I think is a great plus for this tank style. Also the Lulu logo on the old Dance Strap tanks are velvet and in the same color as the rest of the tank, instead of the regular silver reflective one. So I think the older style is more street-wear-friendly. Otherwise there's not too much difference between this one and the older ones.