It's a tricky subject, for sure. There are many ways that you could compare the two products when it comes to their archiving offerings, and of course the landscape is changing all the time with new features being added to Enterprise Vault, and new features being added to Microsoft Exchange.
The truth of the matter is that the 'best' is down to what you want your archiving solution to do today, and where you think it will be heading towards tomorrow.
Here are some good links that compare the two on various levels:
http://msunified.net/2010/11/16/archiving-in-exchange-server-2010-vs-symantec-enterprise-vault/ << November 2010, and compares Exchange 2010 with Enterprise Vault 9.0
http://blog.koenvermoesen.be/2010/06/03/symantec-enterprise-vault-vs-microsoft-exchange-server-archiving/ << Published in June 2010 but then refreshed in March 2012 - a good comparison to read.
http://blogs.technet.com/b/haroldwong/archive/2009/06/19/exchange-server-2010-archiving-and-retention-06-10-09-questions-and-answers-log.aspx << A question and answer session in 2009, and in it Microsoft appear to agree on many levels that the 'best choice' is down to the things I mentioned earlier.