I couldn't imagine you'd get anything else, TBH. An alternative to the 8 Series convertible would be a Bentley Continental or Mercedes SL class, and I'm not even sure Sixt has any of those. I uploaded a photo of the Sixt lot in SFO, and I see a few 8 Series lineups. The photo doesn't include any rear badges, but I can tell what kind of car it is by looking at the front facias (petrol head here).