For licensing details, go to Microsoft, always.

As to performance, in my experience, they're really about the same. There aren't any really insanely radical differences within the query engine between the two versions 2008 & 2012. There are a large number of functional differences, especially around high availability. So the main concern for your upgrade should be which of the two is going to serve you better. Another consideration is support. It's 2013. 2008 is 5 years old. It's going to be taken out of support soon. A new version. SQL Server 2014 is likely to be released this year. If anything, I'd suggest exploring upgrading to that rather than even consider 2008. The amount of labor and testing needed is the same for both, but your support life is much longer for a newer software than for an older one.