It depends on your application and budget. Both the galvanic cell sensor and the optode sensor are accurate instruments. The optode is more expensive than the galvanic cell sensor, but long term can be more stable, and does not require periodic refurbishment and recalibration.  The Oxyguard has a lower initial cost, but does require periodic refurbishment and recalibration by the end user to ensure it is accurate measurements.  Normally for short duration deployments we recommend the galvanic sensor, but for deeper and longer deployments, we recommend the optode.