This is a very open ended question, as it depends on what bank, what OS, and potentially even what hardware.
Some banks are vigorous about the login process and password recovery, others put more value in convenience.
Some smartphone manufacturers are diligent about distributing security updates, but others are very careless and/or have spread themselves too thinly with too many models to maintain.
Some operating systems have a lot more malware "in the wild" and are more prone to user error, others are more locked down by default, and have more thorough checking within official software sources.
Some hardware was manufactured with vulnerabilities, such as with the rowhammer exploit that was discovered to work on smartphones, recently.
There's no binary answer; either could be insecure, depending on the context. It's best to avoid falling into a false sense of security by giving a one-size-fits-all answer. You need to carefully review the security of the routine you're using, and increase it where there are vulnerabilities.