Knockdown is temporary while knock out is permanent. More importantly, gene knockouts will be time consuming and if you are seeking a short tenure project it is better to study the relative effects by gene silencing.
Although si RNA designing is again a specialized discipline but still will require lesser expertise as compared to creating knockouts.
One of the key elements here be the model system you are working on?
If you can povide us with more details of your work we may be able to help you better.
All the Best