Like you said, both series are acceptable at L8, but I would think most girls who make it to Regionals (which is the highest for L8) will be doing the BHS-BHS and that the judges will score a BHS-BHS series higher than a BWO-BHS. If both series were peformed with absolutely no deductions I would think the BHS-BHS routine scores higher.
Now what each gymnast is capable of doing to the best of their ability is up for the coach to decide. Some might think if you hit the BWO-BHS every time you compete, but sporatically hit the BHS-BHS then a coach might want the gymnast to do the BWO-BHS. Regardless of competition, the gymnast should always be striving for the BHS-BHS, at least in practice, because it builds for the next level - where the top gymnasts will be doin the BHS-BLO, not the BHS-BHS (L9).