iOS 9 is probably more secure than iOS 8.4.1 in the sense it's newer and more up to date. Yes, I'm aware the last iOS 9 beta build is technically older then the release of 8.4.1 but the OS itself is newer. I've used my banks app the whole time without issue. The app is a bit slower due to it's not officially updated for 9 yet, but it still functions all the same as it did on 8.