Hi Ana --
Wow, this looks gorgeous on you in both sizes!
I honestly had to flip back and forth a lot between the sizes, and in the end I'm voting for the 8 for all the reasons 50% of the commenters did.
Have you checked what happens in both sizes up top when you bend, sit, etc?
IME even faux wraps gape open a bit when the body is more relaxed, and when they're looser they seem to be worse gapers! I admit I'll be impressed if you can get away without a camisole or fashion tape in either size. [On my rare wrap dress occasion I wear a slip because I'm apparently required to encounter lots of wind....]