I used to play Bg5 a great deal but after a couple of stinging reverses at the hands of Mark Hebden in weekenders round about 1981 went off it. Unfortunately I can't remember the exact method he employed. I know it didn't involve ...a5 and did involve ...f5, ...h5 and announcing mate, although I expect some inadequacies on my part contributed.

I think Smyslov plays 8...a5 because his next move is going to be 9 a3 Nd4. That system benefits in some tactical way from ...a5 being in, although the details escape me (they're in Watson's English series, for example). In general my feeling is that it is a matter of taste. ....a5 is perhaps more simplifying and goes better with the attempts to equalise by playing in the centre. if Black is going for ...h6 and a knigside attack, for example, I doubt he wants to play ...a5.

I personally prefer to leave out ...a5 since I think it just helps white pry open the QS for his heavy pieces.   

As White I play 8.Bg5 because I like to get in the Bxf6 trade before Rb1 for this reason.
The swap on f6 not only fights for the light squares but also lessens any Black KS attacks.
Psakhis-Zili 1996 is a nice model.
 
Donaldson seems to think that ...a5 is not as strong after the Bg5 swap.
In particular, an open a-file means that the ...Bh3 idea is less strong since after Bxh3 Qxh3, the queen usually needs to come back to protect b7 or c7.  In the meantime White can take the a-file and penetrate.
Ulf Andersson-Westerinen 1977 is a perfect example. 

Finally, Mark Dvoretsky wrote this about the ...a5 idea in an analogous position in vol.3 Strategic Play: 
"Sometimes Black plays 10...a5.  This move seems dubious to me, since it runs contrary to a well-known positional principle: 'Don't move pawns on that part of the board where you are weaker'.  The ...a7-a5 advance leads to the opening of the a-file, which most probably favors White - it being he who is dominant on the queenside."


Of course many strong players seem to think its is perfectly fine and always meet Rb1 with ...a5.   
Shirov and Smyslov in particular.  But I still think after Bg5 and Bxf6, the ...a5 push is not as good of an option. 
so I go Bg5 first and only then Rb1, hoping to see ...a5    

I hope this helps you make your choice, but I don't think there is an objectively better move. 

Cheers.

ps- I think its worth mentioning that black can throw in ...h6 at several points prior to move 8.  So, its essential to learn to play Bb2 or Qb3/Ba3 stuff as well as the Bg5 plan.

any opinions on 1.c4 nf6 2nc3 g6 3.g3 bg7 4.nf3 d6 5bg2 nc6 6.00 00 7.d3 e5 8.rb1 vs bg5 question 2 is black better off with a5 in or out?????