Found several good(related) questions here and here but all are nearly a year old.
I will like to know in the current context of Prism development what is better or at l
They are two approaches to solving some-what overlapping scenarios.
Unity shines when you are developing a modular application where users will be using a subset of the modules and modules are self-contained.
MEF shines when you have a lot of 3rd party developers creating plug-ins for your application - or several development teams publishing plugins.
We started out with Prism for core functionality and will be adding MEF for Widget-type functionality later - the idea being that border-line/customerspecific functionality will be delivered in MEF-plugins.
Both frameworks are really easy to get started with (MEF probably a bit 'simpler' from the get-go).
In my oppinion it really depends more on how your development strategy is structured rather than the application in question.