TL;DR: RiteKit API gives you more logos and in higher quality.
Number of logos
Clearbit uses a finite dataset, only improved by self-submitted logos. There are billions of websites that are not in their database.
RiteKit extracts a logo from any website on the fly which means you get one even for the most obscure and new sites.
Clearbit returns logos exactly as they were scraped from the social media accounts mentioned on a site.
RiteKit gets real logos from the site in higher quality and makes their background transparent. This makes the logos look good on any background, not just white.
See https://ritekit.com/api-demo/company-logo (test it without registration or reading documentation).