  The main goal of the next generation of weak lensing probes is to constrain
cosmological parameters by measuring the mass distribution and geometry of the
low redshift Universe and thus to test the concordance model of cosmology. A
future all-sky tomographic cosmic shear survey with design properties similar
to Euclid has the potential to provide the statistical accuracy required to
distinguish between different dark energy models. In order to assess the model
selection capability of such a probe, we consider the dark energy
equation-of-state parameter $w_0$. We forecast the Bayes factor of future
observations, in the light of current information from Planck by computing the
predictive posterior odds distribution. We find that Euclid is unlikely to
overturn current model selection results, and that the future data are likely
to be compatible with a cosmological constant model. This result holds for a
wide range of priors.
