The best PRO of Spring Integration is its very good integration in the Spring environment :-) IMO this is also the only PRO compared to Camel.
Camel offers several different DSLs, you can use Java, Groovy, Scala or Spring XML. I prefer the programming DSLs instead of XML, but that is a matter of taste. Another advantage of Camel is the numerous number of components (over 100 already, including modern cloud interfaces such as Amazon Web Services or Google App Engine).
Thus, I would use Spring Integration in a Spring project where everything else is Spring stuff (if all required components are available such as HTTP, JMS, and so on), and I would use Camel in all other projects.
As already mentioned in the last post, it does not make sense to compare Apache Camel or Spring Integration to Mule, ServiceMix, et al. If you are not aware of the differences between an integration framework and an Enterprise Service Bus, you maybe should read my blog post about exactly this question: http://www.kai-waehner.de/blog/2011/06/02/when-to-use-apache-camel/
Kai Wähner (Twitter: @KaiWaehner)