you should take the course based on what your company is using. If your company is on Classic, then take ADM201, if your company is on Lightning, then take ADX201. One version won't help you prepare more than the other, as the exam is based on concepts and scenarios rather than UI.

For example, understanding the security access model is a concept that is the same in Classic as it is in Lighting. The click path to set up might be different, but the exam doesn't ask those types of questions.

Hope that helps, and good luck on your exam