That question is like asking if one building is more secure than another. It depends on how it was built and how well it is maintained.
If the developers of the website are security-focused (rather than just “get it working”), you’ll see security features including a certificate issued by a trusted CA, 2-Factor Authentication, and enforcement of strong passwords. All of this can be compromised if you have a keylogger or other malware infecting your computer. The most important part here is keeping your computer secure (including browser extension). Make sure you keep up with Windows/Apple/Linux updates as well as browser updates.
As far as apps go, Apple wins IMO. I personally use Android because I love the freedom of the OS, but that freedom has a cost. Android apps from the Google Play store are relatively safe, but there are still reports of problem applications more than Apple. If you side-load an Android app, all bets are off and you take your security into your own hands. The entire iOS ecosystem is a playground with a 10-foot hedge around it. It’s very safe, but you have to pay and play on Apple’s playground equipment. Given that culture and society is migrating to a mobile-first ecosystem, I would expect in the future that apps will become more secure and feature-rich. My personal goal is to have my entire financial life able to be managed via apps on my phone. I’m getting closer. 
Again, make sure you keep your device as up-to-date as possible with both the mobile OS and the apps through the app store.
Phaz