Smoke is a smaller, more focused tool. You will find a lot of things Spray does that Smoke does not. Smoke has a significantly different DSL, relying on Akka's composable Futures to take care of things like error handling.
Smoke evolved through an attempt to build the simplest possible asynchronous HTTP services on top of Akka, without sacrificing any of the key features we needed to deploy those services at scale.
The goal of Smoke is not so much to compete with Spray as it is to provide an HTTP-service-focused alternative to Finagle (https://github.com/twitter/finagle) that uses Akka Futures instead of Twitter Futures, and includes everything we needed to build, test and deploy those services quickly.
So far, it's been working quite well for us.