Choice of meter for writing a treatise: s'loka vs. aaryaa

[Jean-Luc Chevillard]

Dear list members,

being engaged in a study of the popularity of various meters in the 
Tamil speaking world,
I would welcome informations on the reasons (or the background)
for the choice of meter while writing a treatise in Sanskrit.

Are there for instance
articles/books examining, explaining or suggesting reasons why the 
Vākyapadīya was composed in śloka-s
whereas the Sāṃkhyakārikā was composed in āryā?
(I hope I am not mistaken)

Is this simply a consequence of the date of their compositions?

Thanks for any pointers

-- Jean-Luc Chevillard (Paris)