Which Bank Should I Opt to Open a Demat Account as I have accounts with HDFC Bank & Kotak Bank?

None of these are good when comes to opening Demat account. There are many options that are best and you must go ahead with the market champs.

Generally, Demat account is provided by full-time brokers and as well as discount brokerage firms.

Full-time brokers are those who charge huge transactional fees and brokerages, they have charges in every stage of your investments, they even charge for an order you just place and cancel. Banks like Icici Securities, HDFC Securities are not different from them. Sharekhan, Kotak Securities, Ventura Securities, Motilal Oswal are few other examples of Full-time brokerage firms.

The present market pioneers are Discount brokerage firms, among them, Zerodha named firm takes place as best for many reasons.

Zerodha Provides Zero brokerage account. means you will not be charged at all for any of your trades which are more than 2 business days older.
Transactions charges in zerodha reffered as either 0.03% or 20 Rupees whichever lowest. it means even for 10 lakh rupees worth trade costs only 20Rs in zerodha and same would cost 5,000Rs to 5,500Rs in HDFC and Icici Direct.

Zerodha charges are lowest in industry, charges are applied only in intraday trades and never being charged in delivery. Below I compared with HDFC and Zerodha.