That's a tough decision if it is either/or. In time both would be beneficial to have. Both would help you and the shop. A few things to consider would be if the shop suffers more from quality control (or lack there of) or from production. The CWS would help with both. The CWS will help the shop choose the correct processes, wires, gasses, joint configurations, etc., and will also help quality, production and the bottom line. The CWI will help the shop stay code compliant and improve quality and that will help the shops reputation, attract new customers and keep the existing customers happy from a quality standpoint. It will also help the shop with the task of qualifying welders, WPS's and PQR's.

We don't know much about your operation so the responses may be general. How many employees work there, do you have a quality program, is the owner interest or just you, are you an AISC shop, plan on being one, field work or just shop fab, what codes do you work to?

In my opinion the CWS would help the shop more unless the shop suffers from a quality perspective.
The CWS would have an instant impact on production and profitability unless the shop already operates very efficiently.
The CWI would be more helpful to you if you were to leave the shop and seek employment as a CWI. There are more opportunities for the CWI than the CWS but both are very useful and valuable.

If you are not going anywhere I would suggest the CWS first and then the CWI next if the owner (paw-n-law) will pay for it. The CWS will help prepare you for the CWI. Keep absorbing the info and both are good choices to help you personally and the shop. Good luck with your decision and the shop.