SQL Server Migration restore backup vs copy data and log files
For speed (eg less downtime) backup/restore. Backup/restore is quicker because of Instant File Initialisation (only for MDF). The actual backup file is a lot smaller then the MDF/LDF sizes so copies quicker.

It's arguably slighty less risky because there is no downtime on the SQL 2000 instance.

However, detach removes access to the old database. You'll have to take this offline with a backup/restore. If there is a risk of end users connecting, use this (or take offline as soon as backup is complete)

Basically, it's whatever suits you best. I've used both methods for a recent server migration.