These days banking and mobile banking is a default service of most banks, very convenient service.
However, if you would like to do it safe and secure what platform or PC would you take?
That is the question where for now - there is a simple answer: non-jailbreaked(iOS) OR non-rooted(Android) mobile device.
Think: if on PC or Mac or jailbreaked/rooted mobile device - you can have some form of malware or spyware which you even is not aware of,
original iOS or (maybe) Android platform is an exception, up to now, nothing can log your keyboard typing, make remote screenshots etc.
Well,it is so good that for jailbreaking you need to do some and particular dedicated steps and only - if you need to.