Chlorination by SOCl2 vs SO2Cl2

My textbook mentions that chlorination of alkanes by $\ce{SO2Cl2}$ is preferred over $\ce{SOCl2}$, but I didn't understand why. 


Could someone explain which of the two reagents I should use in what situations, and briefly tell me why? 


There is a similar question here: 
[How does SO2Cl2 react with alcohols?](https://chemistry.stackexchange.com/q/33430/21838)


However the answers there did not completely clarify my doubts. 




The reactivity patterns of $\ce{SO2Cl2}$ and $\ce{SOCl2}$ are quite different. $\ce{SOCl2}$ is a good electrophile, and can be thought of as a source of $\ce{Cl-}$ ions. These ions can go on to react in their typical nucleophilic fashion. $\ce{SO2Cl2}$ however is often a $\ce{Cl2}$ source, as it readily decomposes giving off sulfur dioxide. Usually much easier/safer to use this than measuring out (and getting into solution) chlorine gas. The chlorination of simple alkanes by $\ce{Cl2}$ gas (or something that makes it in solution) happens by a radical mechanism i.e. $\ce{Cl.}$ not $\ce{Cl-}$.


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Chlorination of **alkanes** by $\ce{SO2Cl2}$ is preferred over $\ce{SOCl2}$. Just pay little more attention on the word alkanes. So, the statement is absolutely correct. 
Just go through it ([link](http://www.chemistry.mtu.edu/pages/courses/organic/manuals/2411/FreeRad/FreeRad08.pdf))!
Hence, For chlorination in alkanes, if you use $\ce{SO2Cl2}$, It undergoes a free radical substitution reaction and will get the product.




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If we use $\ce{SOCl2}$, it undergoes a nucleophilic substitution reaction. [Here](http://www.masterorganicchemistry.com/2014/02/10/socl2-and-the-sni-mechanism/), this reaction mechanism is explained.


So, You can see here that $\ce{SOCl2}$ doesn't produce a chlorine free radical. Hence, It is not possible to chlorinate alkanes using $\ce{SOCl2}$, which $\ce{SO2Cl2}$ does.


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