Does Force SSL Feature use 301 Redirect

no you dont have to but if you use htaccess and disable it in plugin it will be faster

Thanks @timse201

Would you prefer one over the other? htaccess or allow iThemes to handle it?

i would prefer htaccess because it’s faster
if you enforce www or non www it will first enforce this and after that it will redirect to ssl. so it is safer via htaccess, too.

if you want to use hsts preload you should redirect to ssl first and then to www or you cant get to the list
https://hstspreload.org/