Should i use Limit & Offset in Sql or cursors to handle large data - Mailing list pgsql-novice

Hello,
Just out of curiosity :  On Postgresql ( > 9.0 ) , if i had to iterate
over the table with 3 million records , which method uses less memory &
cpu usage but faster : Using cursor and fetch or just plain SQL with
offset and limit ? And why is the difference ?

Thanks.