I prefer the first.
Interested in Language
1.Opel is not as expensive as BMW.
2.Opel is not as expensive as BMW is.
I know that the first sentence is correct. What about the second?
That sounds to me as if you are talking about the value/cost of the whole company. I would use "Opels are not as expensive as BMWs" or "An Opel is not as expensive as a BMW".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.