In this SQL Server Pro article Save Thousands in Licensing Costs for SQL Server AlwaysOn Availability Groups Michael Campbell makes a case on why Windows 2012 Standard edition provides the best bang for buck in supporting your AlwaysOn Availability Group implementation.

Interestingly he also highlights some reasons why he thinks the older Windows version, Windows 2008 R2 Enterprise Edition, is the not best choice when it comes to implementing AG groups.

Though in my experience I have implemented/supported a number of AGs on Windows 2008 R2 Enterprise Edition without issue, however, I did need to apply the relevant hotfixes.