I am not familiar at all with Bixby but I own 5 Echos and I can say from first hand experience that Alexa is much "smarter" than Siri.
For starters, the speech recognition on Alexa is better than Siri. I would estimate <1% of the time I have to repeat a question to Alexa but >10% of the time I have to repeat a question to Siri.
But I think the bigger difference is that Alexa was originally designed to be voice response only (no screen) while Siri was originally designed to work with a screen. Whenever I ask Alexa a question, the vast majority of the time I get a specific answer to a specific question and on rare occasion I get some variation of "I don't know that". With Siri, at least half the time I ask a question I get a response of, "here is what I found on the web" and a list of links appear on my iPhone screen which allows me to interact with some links to try to find what I was looking for. If I am in a situation where I can look at my screen and use my hands, this is may be helpful as compared to having to type a question into a search engine. However, I would greatly prefer Alexa's approach of telling me the answer she found rather than helping me search for it on my own.