It depends on the situation. By far, the migration utility is much faster. However, it migrates your existing database warts and all.

The imp/exp method allows you to fix wrongs created by others. You may need to change the db_block_size or you may need to resize objects. However, depending on the size of your db, you might be waiting a long time.

I have used both extensively. Both methods work well for what they are intended.