Your online banking either through your computer or smartphone is equally secure and insecure depending on how you manage your information and what you do with your computer. If you are the type of person who likes to download illegal content from non-secure sites, you open your computer to the possibility of downloading worms and key-tap programs which allows fraudsters to discover your information.
Likewise, if you use an App, a lot of the time your user information is saved and if you have an easy to guess password, then anyone who picks up your phone or knows the mobile banking text information (saved in your contacts maybe?) can have access to your account information and balances. Some banks are even piloting having your credit card linked to your smart phone so the check out worker will literally scan your phone's screen instead of having to carry a wallet.
So it boils down to this: Do you download illegal content and/or have a very good anti-virus anti hack program on your computer? Do you have a password on your phone and do you save your info on your phone.
If you are diligent in protecting your personal data, both mobile and app based banking can be very secure as can PC based banking. However, we all need to be aware that ever day the fraudsters are getting smarter and smarter and the best defense against fraud is to change passwords frequently and to review your account information often.