There are two distinct differences in these two areas. SEO, done well, will give you a long return on your investment, giving you results for longer. Paid advertising like a Google Ads or banner sponsorship will only work while you’re paying. Good SEO will keep you on top of the search results for a long time but you won’t necessarily see results right away.
Spend where you feel works for you. If you need leads and sales right now, use on paid ads more. Investing in the long term with SEO? Spend more there. A 50/50 split might seem a sensible starting point for you, or you may well go 80/20.