Both are BDD (Behavior Driven Development) capable unit test frameworks for Scala written in Scala. And Specs is built upon may also involve the ScalaTest frame
IDE support may be another point
I've been trying to get Specs to work with Eclipse through JUnit, and I found the official solution to be a bit "hacky". Specs setup: http://code.google.com/p/specs/wiki/RunningSpecs#Run_your_specification_with_JUnit4_in_Eclipse
ScalaTest's integration (also through JUnit) with seems a bit less hacky. Still, I haven't got any of them to work as well as JUnit and Java.
ScalaTest setup: http://groups.google.com/group/scalatest-users/web/running-scalatest-from-eclipse