10 years ago, I only used free SEO but recent years, I used both paid and free SEO because I didn't have more free time for free SEO. Beside, free (Unpaid) SEO just gave me normal backlinks while paid SEO gave me quality backlinks without spending more time and efforts for them.
When a search engine returns its search results, it gives you two types: organic and paid. Organic search results are the Web page listings that most closely match the user’s search query based on relevance. Also called “natural” search results, ranking high in the organic results is what SEO is all about.
Paid results are basically advertisements — the Web site owners have paid to have their Web pages display for certain keywords, so these listings show up when someone runs a search query containing those keywords.
On a search results page, you can tell paid results from organic ones because search engines set apart the paid listings, putting them above or to the right of the organic results, or giving them a shaded background, border lines, or other visual clues.
Paid SEO in any case.
Because they make efforts to bring your site to the top position in Google.
With free SEO you cannot guarantee that you have used up all the efforts to rank your website to the top spot on Google.