Ї вќжу Сђшу чђще Мђши (Gen.).	I see Sasha oftener than Masha [sees him].
	or: I see Sasha oftener than [I see] Masha.
Ї вќжу Сђшу чђще, чљм Мђша (Nom.).	I see Sasha oftener than Masha [sees him].
Ї вќжу Сђшу чђще, чљм Мђшу (Acc.).	I see Sasha oftener than [I see] Masha.
If the second item does not have a Nominative or Accusative function in the sentence, you have no choice: you have to do it the easy way, i.e. with чљм (see below). In fact, you might want to do it the easy way all the time, which is OK, but you should know the Genitive way because you will hear and see it often.
В Сђшиной грџппе говорїт быстрље, чљм в нђшей грџппе.	In Sasha's section they talk faster than in our section.