From the security perspective both snaps and flatpaks are preferable to dep/rpm for browsers, email clients, office suite, document viewers and other stuff that is used to parse untrusted data often (due to [wip] sandboxing and auto update).

Snap packages are better maintained (more often with direct involvement of the app developer) and generally receive updates a bit earlier than flatpak. In both cases you need to pay attention who the app maintainer is and I'd argue that in case of unknown maintaners deb/rpm packages are safer choice.