Use the Data Loader when:

– You need to load 50,000 or more records.
– You need to load into an object that is not yet supported by web-based importing.
– You want to schedule regular data loads, such as nightly imports.
– You want to be able to save multiple mapping files for later use.
– You want to export your data for backup purposes.
– Use web-based importing when:
– You are loading fewer than 50,000 records.
– The object you need to import is supported by the web-based import wizards.
– You want to prevent duplicates by uploading records according to account name and site, contact email address, or lead email address.