I vote Laxus... definitely not Erza
Seriously, I'm not even being mean to Erza. Erza can be a good member of Fairy Tail, but she won't make a good master of Fairy Tail.
In my opinion, Erza as master would be similar to Mavis being master. And I think that Makarov >>> Mavis at being master.
Laxus is more similar to Makarov while Erza would be more similar to Mavis.
In my opinion, Laxus has pretty much all the necessary qualities of being a master.
He is strong/powerful enough, he has authority, he has brains, and he seems to value friendship (this is a rather more recent change in his personality).
Erza has pretty much most of the qualities that I listed above except that she can be indecisive and emotionally biased, which Laxus seems to be good at avoiding.