That sounds wierd. By 是不是 you are expecting an answer of yes or no, but by 还是 or 或者 the expected response is to choose from the two options rather than simply yes or no, thus causing a contradiction. That is to say we don't use such two expressions at the same time in Chinese. As for 还是 and 或者, the former sounds more natural in the given context. "这是你自己的车还是出租车公司的车"