In our experience here (which is mostly UK-only) users will type in a sub-domain, however they will leave off anything after the domain (like folders). The forward-slash is cumbersome in use as general site visitors get confused with back slashes and just visit the main site to try and find what they want - completely ignoring the rest of the URL.
Having said that though - we also have a glut of users who, despite instruction to the contrary, will always type in www. before any domain that is given.
Our simple advice would be to use sub-domains but also create www. sub-domains hanging off each entry.
Just using folders gives off the false impression that there is little isolation between customers. Also sub-domains will allow you to vary by IP address so that if certain customers demand more resources they can be given isolated servers, etc.