Advice needed

Hi,
I have a WP 3.4 blog and a web site (html – no wp) – I would like to convert the web site to a WP installation, using another theme, other plugins(pictures gallery,etc.) – additional users, etc.
This would avoid having to maintain two WP installations.

Do I need to run Multisite, or is it better to perform dual installations ?
Thank you
Jean-Louis