Big 4 will give you a good experience in terms of communication and knowledge, however I have seen many of my friends not able to get sufficient time for final exam preparations thus only few out of many were able to manage to clear in 1st attempt.
If you are planing for Job after becoming CA, it will definitely add value to it.
However if you want go for Practice, mid size firm will help you to develope independent attitude for handling complete task. Also you will get some time for your final preparations.
as you can Imagine, to apply for both groups in final you need 1.5 year time for class and 6 months time for self studies to cover such a huge syllabus.
But do take decision carefully, as you are planing to spend your next 3 years at a specific place. So suggested to contact your seniors and respective sirs.