According to the example on p. 10 of LEED Rating System Selection Guide (June 2011 version), LEED for New Construction is the best rating system for cases where the majority (but not the totality) of the interior fit-out is installed by the owner.

Example:
«A new office is being constructed, and the interior fit‐out is being completed with the exception of furniture installation and interior paint application for the entire gross floor area. LEED for Core & Shell would be the result of a strict interpretation of this guidance because the fit‐out is not technically complete for at least 40% of gross floor area. However, LEED for New Construction is the best rating system for this project because the vast majority of the components that make up the interior fit out are complete.»

We are now working in a building that is very similar to the one described in the above example. It is an office building where 80% of gross floor area will be occupied by tenants. Leasable area consists of open spaces where technical flooring, ceiling, lighting and HVAC systems as well as main partition walls will be totally installed by the owner.

Future tenants will receive their offices ready to use (in exception of the floor finishing). However they can add partition walls to customize the open spaces layout to their specific needs (as well as, of course, furniture).

The question is: can we choose between New Construction and Core & Shell or is the use of any of these systems mandatory (and, of course, which one)?