It's great to deploy both SEO and PPC simultaneously as eventually you will get the maximum conversions. However, if you are limited on budget and want quick results, it is best to start with PPC. You can understand which keywords convert and show your C-Level executives conversions in a short time period. This can help build trust between you and the agency to increase the budget thereafter. Consider starting with SEO first if you have a long term view and have sufficient budget as it takes time and has a compounding effect. Read this article to understand why you should be using both channels together.