Booking.com has way more properties but does not always properly vet them. Its easy to find that you've booked a property but the property owner is miles away and does not speak your language (i.e. English). That said, their customer service often resolves things.
In my limited impression Airbnb has gotten better at customer service but has basically pivoted out of their core business model (i.e. providing short term stays) and at the same time provided huge incentives for people to mislead people with respect to the listing. They are consistently the hardest to verify (i.e. that what you are reading or seeing is real) and I would really only use them for very boutique properties that can't be found on other platforms.
In some ways it just depends on how much up front time you have to do your own vetting and how much risk you are willing to tolerate for a potentially experience. AirBnb is high in both categories with results that very rarely are worth it to me.