In case you are getting a good opportunity in BIG4...join it without much thinking.
Further it is not about small or big firm. It is all about good exposure in Tax, Audit, finance field.
I would suggest you to join an old and medium CA firm. I would suggest you to gain good exposure in tax and finance field keeping in view future career prospects.
You are requested to explore the links given below to locate CA Firms for articleship near your place. Please go through the given links as they are really useful: