Using either Simple DB or Dynamo DB totally depends on your requirements,
One of the differences used to be in indexing. DynamoDB used to limit you to creating indexes at the time of creating the table. However, now (since early 2014), there is the concept of Global Secondary Index (GSI). The GSI can be created on the table at any time. Upto 5 are supported. So, indexing is no longer a blocking issue for many use cases.
You should also know that SimpleDB has size and performance limits. (10GB and say, 25 requests/sec)
Perhaps eventually, DynamoDB will replace SimpleDB in all but the most simple use cases.
Hope this helps