1. The direction of the data load depends on the approach that was adopted while designing the Warehouse and the datamart. If the top down approach, which Kimbal mentions is used, then data will be moved from DW to DM, whereas if Inmons approach of bottom-up design is used, then data will flow from DM to DW.

2. There is nothing like which one is a better option. Basically the decision of loading data whether from DM to DW, or from DW to DM depends on the design as mentioned above.