Entity Framework unitofwork vs self-tracking entities

Question

This questions stems from a n-tier application. (non-WCF - only separate assemblies)

Should I use a unitofwork pattern and keep my object context around for entity change tracking?

or

Should I use self-tracking entities and always dispose my object context after a query, insert, delete, etc... and have the entities track themselves?

If I go with the second question do I need to use POCO?

Thanks!