Use multisite for the following reasons:
1.) You can have the same login credentials for each site, so users that need to get into admin don’t need to remember what site credentials to use.
2.) Depending on how your sites are setup a free plugin like “WP (three?) Broadcast” can let you copy pages/posts from one site to the other (including featured images)
3.) Site updates to plugins and themes all happen in one place at the network level
4.) Less space required on the file server because you have only 1 instance of wordpress installed; no duplicate files.
5.) Cheaper. Most themes and plugins you pay for are based on the number of wordpress installs you have. In your case it will always be 1 for this client. So basically you save on licensing most of the time. The exception is when licenses are based on “domain”
6.) Use WordPress “MU Domain Mapping” plugin to make multisite management easier.
Additional Note: In multisite the sites will technically be seperated both the media library and on posts/pages. This is by design in wordpress, but as i stated there is the broadcast plugin that will let you share articles across domains and subdomains. With wordpress domain mapping plugin you can setup sites like site1.com, site2.com or foo.site1.com, bar.site1.com or you can mix and match as needed. So site1.com, foo.site1.com, site2.com, bar.site2.com, etc.