Lesser known is actually worse. Less work by the community on removing security holes vs an attacker who can concentrate efforts.
Not that it matters - high profile should not use any kind of phone for anything serious.
The most secure option would be not to carry a mobile phone at all. Otherwise, iOS is probably the most secure mobile operating system, simply based on the price of remote exploits: a remote exploit for iOS is worth several times more ($500K-1M) than the equivalent one for Android ($100K-200K).
that seems like a small difference in cost for high value data...
dumb phones generally don't have the capability to do end-to-end crypto or full disk encryption and other desirable things.