I think it's not really about which one is "better", but understanding that each app negotiates differently with hotels, so prices really vary a lot. What nobody mentioned is that it's worth checking the cancellation policy for each one before you book - because sometimes one app charges you a fee to cancel and another lets you do it for free, and that can make a difference to your wallet if your plans change. In your case, before deciding which app to use, I'd check the same hotels on all three and compare them directly.
But there's an important detail: their customer service is pretty different. Booking has that chat that works 24/7, Decolar also has decent support, but Airbnb is more of a hassle if something goes wrong because you're dealing with the host and not a big company. So if you want the peace of mind of having someone to call if something breaks in the middle of the night, Booking or Decolar come out ahead. For a quick trip to Bahia where you just want to get there and crash, what I'd really recommend is searching all three, writing down the prices of the hotels you liked, and then pick whichever has the best price at that specific moment - because the difference can be like 50, 100 reais depending on the day.