It is a good question.
Let me tell you the difference between preparatory and language course, please remember that both courses can lead to degree. But there is one difference.
In preparatory course you will study some other subjects along with Russian language and in Russian language course you will study only Russian language.
If you will clear preparatory course then after completion you will be transferred directly to degree without any exams but you need to clear the exams of preparatory course. (it is always hard to study three subjects in RUSSIAN language when you are foreign student, so mostly people clear preparatory course in more than one year )
Whereas in Russian language course, you need to give only language test, after finishing that course in order to get admission in to the degree. But in both cases you can take admission in degree course. Only the thing is you have to decide which course is better and easy for you.
Thank you for understanding