When should I use Kepler instead of Taverna?

Three big differences between Kepler and Taverna are (1) the user interface, (2) the models of computation and (3) the user communities being served. If you want to make sure, just send an email to the Taverna or Kepler mailing lists for feedback.

1. Taverna user design the workflow via a tree-based interface, with the workflow graph being rendered automatically based on this, whereas in Kepler a user draws a workflow manually, as one would do with electronic circuit design.
2. Kepler works based on the concept of 'directors', which dictate the models of execution used within a workflow. Taverna uses one particular model of computation, based on the lambda calculus (see TavernaSemantics), whereas in Kepler users have a range of models of computation available. See a joint paper describing which directors are available in which combinations within Kepler.
3. The Taverna team has primarily focussed on supporting the life sciences community (biology, chemistry and medical imaging), whereas in Kepler the emphasis so far has been on ecology and geology.