Ruben,
If you set log mode to overwrite, you won't be able to do a point in time recovery. This shouldn't be used for normal operation.
The better option is to leave log mode as normal, but either move backups to another lower cost storage solution or purge backups that are no longer needed.
Checkout the Housekeeping for Backup Catalog and Backup Storage section in the admin guide: http://help.sap.com/hana/sap_hana_administration_guide_en.pdf
