Regarding articleship

Hi.. i have just cleared my ipcc this attempt (3rd attempt).. i live in coimbatore. Now i have to take up my articleship. i dont know whether to go for big4 or a medium sized firms. i have also registered for icwa (inter). how many days will be required to clear icwa? 

also i heard that in big4, v cant do auditing if v choose taxation dept and vice versa. is it true?

pls suggest me some good firms..