Eles são sinônimos. Veja as citações da documentação do mod_rewrite abaixo:
Sobre as variáveis do servidor, como HTTP_HOST
:
These variables all correspond to the similarly named HTTP MIME-headers,
C variables of the Apache server or struct tm fields of the Unix system.
Most are documented elsewhere in the Manual or in the CGI specification.
Sobre HTTP:variable
:
%{HTTP:header}, where header can be any HTTP MIME-header name, can always
be used to obtain the value of a header sent in the HTTP request.