What is the benefit of submitting to a special conference session instead of the regular track? Assuming that I want to submit a paper related to the topic X to conference Y in CS, and there is a special session in that conference which covers the topic X; in that case, is the best choice to submit the paper to the special session? 

Well, as far as I know, the special sessions have a small pool of submitted papers around similar themes, from which they need 5-6 papers to accept. So I was wondering if I would lower down my chances by submitting to a SS as the pool becomes more competitive for me?
It depends on the conference. Sometimes, "Special Sessions" are organized by an individual to "fill the program" and the organizer will try to find people willing to submit to this session. In those cases, you might even be asked to submit your work for the session and review might not be overly tough.

In other cases, it might happen that the special session is overcrowded - in such cases you might have a hard time. 

Nonetheless, a good paper chair will move your paper to the session which is most appropriate - so it might happen that you submit to the special session and end up in a common track, or vice versa. 

One good thing about special sessions is, that they draw the audience interested in the field, and they are a good opportunity for networking and exchanging ideas. Therefore, I would recommend selecting the special session.