Not the op, but I use both. Firefox focus as default (any link from a random app open on a short lived session, it just feels right), for regular browsing Firefox mobile (and yes, if I'm there and I choose to open a temporary tab, I use the normal incognito tab, with ublock origin)
> any link from a random app open on a short lived session
My reason is mostly the above (and other related security reasons). In particular the ability to clean sessions, cookies etc. just by hitting the trash can next to the URL bar.
Secondary reason is on a small screen usability. The simpler the better. Firefox Focus is simple, minimal settings, and of course quick and easy to reset sessions.
I do that as well. One problem I have is that if I send a tab from FF desktop to FF Android it opens in FF Focus, although it's basically a click from a FF Android notification.
I tend to find Firefox Focus preferable on mobile anyway.