Prism v4: Unity or MEF?

None is "better": they are different things.

IMO your choice should be driven only by your requirements. Based on the requirements you posted here I would suggest you to use MEF, because you have modules contained in DLLs and the main app is unaware of modules to load. Those tasks are the reason that why MEF exists.

Anyway you could use them both: MEF for modularity and Unity to take advantages of dependency injection (testability, reusability, ...)

If all the modules are not recompiled at the same time as the app, then MEF gives you lots of ways to cope with changing interfaces in the main app. Otherwise MEF may be more complex then you need.

I've been using Unity over a year with PRISM but I've noticed some serious memory leaking issues. Hence I decided to give PRISM 4 and MEF a go. What I've done is firstly converting my app to use PRISM 4 with Unity. Then I converted a branch to use MEF.It may sound funny but MEF seems to handle memory consumption and release somehow better than Unity.

Would be nice to hear whether others have made the same experience?