It is definitely true that the majority of app producers update google before Amazon. I have seen Amazon as much as 3 releases behind. I've also seen them as little as a couple of weeks behind.
It's just a matter of how they prioritize their releases, and each app producer makes their own decisions on that.
And the free apps are nice.
I use both app stores on my Nexus and Samsung. The app store updaters know which app came from where and will tell you so if you try to update one with the other.