Is creating a long password for my coinbase account enough to keep my bitcoins secure?
The long password will not help that much and is also likely difficult to enter. Much better enable 2 Factor Authentication with Coinbase (https://support.coinbase.com/customer/en/portal/articles/1658338-how-do-i-set-up-2-factor-authentication-) which gives you a second level of control. 

If you have the wallet e.g. on the phone you have much more control on your funds and do not depend on the security implementations of Coinbase. Also does Coinbase have limits when moving funds which does not apply to your funds in your local wallet.

Often splitting funds in local and online wallets gives most flexibility and security so your eggs are not all in one basket.