As a seasoned Spring user I was assuming that Spring Integration would make the most sense in a recent project requiring some (JMS) messaging capabilities (more details). Af
It really depends on what you want to do. If you need to extend something to build your own messaging solution Spring Integration has the better programming model. If you need something that supports many protocols without custom code, Camel is ahead of Spring Integration.
Having a small scale shootout is a very good idea, just make sure you're trying to do the type of things that you'd typically be doing in the project.
--disclaimer: I'm a Spring Integration committer