Active Directory Certificate Services upgradation suggestion.

Hi

We are preparing to migrate our enterprise PKI infrastructure from Microsoft Windows Server 2008 ADCS to Microsoft Windows Server 2008 R2 ADCS.I have some questions regarding upgradation though?

1. Is there any challenge for in-place upgrade ?what's the pros and cons?

2. What is the challenge if we do migration to Server 2008 R2, i mean "backup and restore".What's the pros and cons?

3. What is the challenge if we migrate to Server 2012 .What's the pros and cons?

Out of above three scenarios which can be the best ?

Thanks