I don't have much experience with google but between Alexa and Siri they go blow for blow.
This ranges from capability, accuracy of dictation, quality of the answer, etc etc.
For example, on a Saturday night I set a reminder for "tomorrow" however it was 2 am.
Siri "Do you mean later today (which I did) or Monday?"
Alexa "Ok I'll remind you" and sets it for Monday
That is essentially user error but the fact that human error is accounted for is a nice touch.
Or certain information accuracy. For example if you ask the distance to the moon (which varies) you'll get two different answers between Alexa and Siri. Siri is fairly up to date and accurate. Alexa gives you the mean, which isn't what I ask and it doesn't tell me its the mean.
Conversely Alexa is better around the kitchen, recipes, multiple timers (comparing to HomePod), substitute ingredients.
I would say they are as good and suck equally just need to find which one works with you personally better.
But Siri in generally works great for me, the HomePod can pick up me talking normally for distances people would struggle with so for HomeKit its awesome. On iOS it took some training to get my voice down but it works well enough, I use it for making calls, sending messages and getting directions while driving. Plus A LOT of math, it can answer a math question much faster than I can unlock and open the calc app.
I wont deny there is still plenty of room for improvement with Siri. Words that sound similar to other words she'll lean toward the commonly used one which can be very frustrating. Conversely I find it amazing how on iOS its at the point where she only responds to me, no matter how hard someone tries.