If it comes to theamwork i think saber has the adventage because natsu and gajeel only fought once or twice together but rogue and sting are famous for being the dragon slayer duo of sabertooth.
http://www.mangareader.net/fairy-tail/258/20
So if it is about teamwork this would mean...
Totaly agree with your guess i also think that mira was chosen but the others asked her if they could go instead of her and by others i mean not only gray :)
Even the master thinks that mira would be the better choice which u can read here:
http://www.mangareader.net/fairy-tail/265/6...
