We had to get our app out on time because our release date was inflexible. SR520 tolling had started, and the commuters were going to hit it as the new year (2012) rolled in. We wanted to be there to help the commuters make an informed choice when they had to cross lake Washington.
Google market promised to publish the app instantly as long as we adhered to their ToS. And, our app did.
I had heard that Amazon market took about a week (or more) to review and approve the app. [We were having the same issue with our iPhone app. It was the first one out of the dev gate, but had stalled in the iTunes approval process. Thanks to the holiday season and all.]
Timing-wise, it made a lot of sense to go with Google market.
We wanted the users to have easy access to the app, and wanted a seamless install process.
All the Android devices (phones) I have seen come with the Google Market app installed on them. As for the Amazon Appstore app, one has to install it manually. I am sure the Kindle Fire will come installed with it, but we were targeting all the Droid phone owners.
Based on all the apps I have installed from the Amazon Appstore, they seem to suffer from the 'ET phone home' syndrome. I for one, do not log into the store all the time. So, the apps stop working when they cannot call home. I understand that some of the paid apps are licensed on the server, and they connect to the mother ship to verify the license. But, what about free apps? Why do they have to connect? Download new ads? We didn't want our users to go through this.
Android dev license fee is one time $25. Amazon AppStore subscription is $99/yr. But, Amazon waives the first year fee, to lure new developers. So, cost was not a driver in this decision.
Google Market can publish apps around the world.
Amazon Appstore only allows publishing within the US.
This really didn't matter for us since we were hyper-local (Puget sound only). But, our Droid carrying friends in India and other countries could not have enjoyed (tested) the app if we were on Amazon.
I do know that Amazon Appstore has other advantages: Ability to use Amazon account to pay (1-click). Ability to purchase once and use on multiple devices using the same credentials. Amazon does market the apps by offering them free for a day. You need to be lucky to be picked, though. This has its own pros and cons.
Of course, with the Kindle Fire, there is only one place to buy your apps - Amazon Appstore. So, if you want your app on the Fire, then you better publish on Appstore.