Just been looking at the comparison between WordPress and Symphony at Compare Content Management Systems - cmsmatrix.org - The Content Management Comparison Tool and this gives a good overview of the differences and apart from the XSLT, designing a skin\template will be very similar in both app's.
The matrix also shows that there is commercial support and training with Symphony so is the client prepared to pay for this?
If the job is to big or not within your skill range and the client doesn't want to pay for you to learn, can you not take the job, pass it on to a developer that has the experience and make a middle man cut?