It depends on the circumstances.

MODs are best for linking small collections of elements that may be repeated within a project or reused in various projects. Examples of this would be unit plans for multi-family developments and standard lavatory layouts respectively.

Linking PLNs is preferable for entire floors or buildings which are subject to frequent change and updating. An example of this would be linking multiple buildings being designed for a single campus. I have also used this approach for linking the architectural and structural models of large projects.

The main difference is that PLNs bring with them all their attributes (layers, materials, etc.) while MODs contain only those actually used by the components of the module.