I've found Cloudflare is cheaper than Google Domains, and allows for more flexibility of setups. A $12 domain on google can be $9 or less on Cloudflare. (this is a per-year price. So the cheaper you get the domain, will exponentially be cheaper over 10+ years)
Cloudflare is also a layer of protection to your pihole. You shouldn't run in to that many issues by buying a domain but I'm not sure what you're trying to do. Just buying a domain is like buying a username, it doesn't do much until you point it somewhere. So if you're pointing it to your pi-hole to host something then I'd look in to hosting services to save yourself a bit of a security issue.
Ah okay, then yeah for sure I'd say there is really no difference between them besides price and convenience of redirecting your DNS. You can always transfer where it's registered later as well depending on if they charge fees or not.