"Which" is the right choice because you're choosing between a limited number of options. But after the brand names, the word "car" is redundant. You would only need to specify if you meant some other kind of vehicle like BMW motorcycles or Toyota trucks. I think the most natural way to ask would be, "Which is more expensive- a BMW or a Toyota?"

The second one, but I would say ... "Which *are* more expensive, BMW cars or Toyota cars?" Since cars is plural, use 'are'.

Actually, two possibilities, but first note we really wouldn’t use “cars” at all. So . . . “Which are more expensive, BMWs or Toyotas?” Or it could work in the singular just as well: “Which is more expensive, a BMW or a Toyota?” It’s understood that we’re talking about cars. If you wanted to specify to a (highly uninformed person) that BMWs and Toyotas are cars, you might say: “Which cars are more expensive, BMWs or Toyotas?” Note one other detail that many native writers make a mistake with: with the plural, there is no apostrophe after “BMW” or “Toyota.” This is a very common mistake, almost to the point that the usage may change the rules someday. But currently, with standard written English, you’d correctly only use the apostrophe to signal possession. E.g., “A BMW’s gas mileage is not as good as a Toyota’s.”

Definitely the second

Neither sentence make sense because there are many kinds of Toyotas and BMWs, with different prices. So it isn’t clear what you are comparing. If you’ve talked about specific models, it would be clear and precise to say Which is more expensive, the BMW or the Toyota? (Because you’re comparing a limited number of choices) However, the question ‘What’s more expensive?’ is also very natural and native-sounding (It just doesn’t emphasize that there are a limited number of choices) What’s more expensive, flying or driving? (Even though you only mention two possibilities, it’s part of a broader discussion with implied other choices. It sounds natural.)