rate of dep as per law or estimate?

You can follow with law that is best for accounting purpose

I think according to AS 6, you can follow 20%. A rate which is lesser than the rate in Companies Act is not passible to follow.

Disclosures are required.

The rates of depreciation  prescribed in Schedule XIV are the minimum rates. No company can charge depreciation at a rate lower than prescribed in the schedule. If, based on technical estimate, higher rate is justifiable, the Company can apply such higher rates for the purposes of computing depreciation under Companies Act.

mr. Pavan is absolutaly right

 As regards the company, Before payment of dividend when profit is required to be calculated as complient with Section 205, one has to adopt Schedule XIV Rates are to be adopted. 

A company may depreciate asset at lower rates not without disclosure of these material departure from Accounting Standard 6, which is again debatable.