MBZ380se I don't know about the american 500 but the my 500 was 97 octane (93 in USA) My parents ran it on 87 for many years never did any harm but it enjoyed 93 when I had it.
personally the 300se-sel is to slow for me
They also come with less options standard but by 90-91 everything was pretty much on the w126's
I would go with the 420 out of the 2
my 2 cents