SQL Server 2008 on VM

Hi,

We migrated our sql server instances that are mix of 2005, 2008, 2008 r2 (standard, enterprise) from physical to virtual. We have UCS in our data centre. In VM environment we are sharing CPU. Our licenses based on per cpu.

My question is:

For any new sql server or instance installation, should I use standard or enterprise edition of sql server. Which one will help to save money? I think enterprise edition is move expensive but gives benefits in vm environment when we have multiple instances on one server (vm).

Please help.

Thanks