AirBNB is super annoying. You have to agree to meet at a certain time. You have to hope they accept your booking. While booking you cannot sort by price, etc. AirBNB informs about demand causing prices to go up in popular periods. Then for most places you basically have to put in writing if you could pretty please rent the place. This while on any hotel you're accepted as long as the credit card check goes through.
I often book by first searching via Booking.com, then trying to book directly or searching for best rate.
I don't travel too often, but AirBNB being cheapest is not my experience. Maybe cheapest no matter the quality and convenience? A shared room is not the same as a hotel for instance. I don't want to feel bad about coming back at e.g. 3am. In a hotel that's pretty much fine. With AirBNB I stick to renting out the whole apartment and for those the prices are slightly above hotels prices (these apartments are often tiny btw).