The reason to use something like Associate-o-Matic is that you host it on your own server and can customise it as much as you like. Astore is too simple really, you are stuck with the basic form and appearance and you cannot do ANYTHING with the code. With both solutions it's going to be difficult enough to get sales (FAR too many Amazon-based sites around already), but at least with AOM you have a chance to change things, including both the look and the code, and it's on YOUR domain, not tied to the Astore one.
Do both, create AOM shop and then random show astore in your website. then you just promote only AOM like me. backlink is the key