Without having the same context as you have it is difficult to provide an informed answer (which I’m sure you understand), but I’ll do my best to express my point of view.
One of the challenges that Prism was built to address is
Multi-Targeting. That is facilitate the creation of applications that share the most of their code base, between WPF and Silverlight. The guidelines provided in the Prism documentation about
this, the project linker, and
this guidance you should be able to create a WPF application that can be “replicated” for Silverlight without many issues.
Something else that you might want to consider is the
YAGNI principle. Unless you are completely certain that the Silverlight application is going to be used, you could avoid developing for Silverlight as well. Of course, you would need to contrast the possible
risk of having created the Silverlight application with no need, with the time/money differences between the development team tackling only WPF or WPF & SL at once.
Please let me know if this helps.