Which voice assistant is better for Home Assistant?

Alexa definitely has the best devices Echo Spot, Echo Show etc.

How does Alexa behave with questions or does Alexa want to sell something to you more often in responses?
Does Alexa understand you very well?

Alexa is supposedly worse in general knowledge

How does Alexa behave with questions or rather

Identical to the other providers.

does Alexa often try to sell something with its answers?

Practically only if you have something in your Amazon shopping cart and there are price changes.

Alexa is supposedly worse at general knowledge

Wiki entries.

Alexa definitely has the best devices Echo Spot, Echo Show, etc.

Yep!