My thoughts are;

Indoor levels---Blocks,wmp's and models
Blocks for level structures, wmps for doors etc, models for assetts.

Outdoor, open scenes---Hmp, models, blocks, wmps.
Majority is models with blocks and wmps for structures requiring collision.

Outdoor, limited scenes---models.

Or --- models and collision blocks.

The assumption is that you are using A6 or 7, latest updates and have an oprational scene managment script.