Always go for mid-sized firms. It has numerous advantages.
First, you'll work across-the-board throughout the spectrum. You'll learn, basic accounting, full-fledged auditing, taxation, both direct & indirect, company law, etc.
In our firm, each article is assigned a few particular companies which the article leads throughout the year. The article assigned is responsible for everything that concerns the auditee-company, from basic accounting, consulting on their accounting policies, auditing from vouching to finalization, tax audit, computation of income, and finally company law, i.e. ROC forms and stuff.
If you work for a big-firm, you'll be assigned a specific role, for instance, audit(it's almost always Audit). And even in Audit, you'll be assigned a specific part of the Company's affairs, for example auditing for Fixed Assets/ Current Liabilities/ Loans etc. While working at a big firm does make you a tad-bit more professional, your communication skills would definitely improve, you'll pretty much live a proper Corporate-life. But apart from that, there isn't much you'll be offered.
I've had the privilege to talk to a few recruiters in the past, and most of them unanimously agree to the fact that interns from Big4/other big firms are only well-presented. Interns from mid-sized firms, on the other hand have adequate professionalism, such that it does not offset the balance, but they do outcompete their Big4/big-firm peers in terms of knowledge in all the fields.
So, well, always go for a mid-sized firm, in my opinion.