Depends on how long your going to be using EV and such as well.
We went through a simlar exercise and I found depending on the Total Number of Mailbox to decide if Perpertual over Foundation was better.
But assuming you for Perpertual
If you got with Enterprise Vault Advanced Management and and E-Discovery standard
This gets you
Mailbox Archiving
PST Migration
Vault Cache
Public Folder
Journaling
Discovery Accelerator
------------------
If you got for Storage Management for Exchange Enterprise Edition
You Also get
Sharepoint
Secure Messaging
ECM
File System Archiving
IM Archiving
Forgot to mention
Also depends on what e-discovery components you want mentioned the standard which is Journaling and Discovery Accelerator but there is a Advanced and Enterprise Edition
However assuming you went Enterprise for Both it is cheaper to buy Foundation I believe as the maintainence costs over 5 years doesn't rep profits with standard or Advanced it is possible to make about $70,000 saving over 5 years as for years 2-5 your only buying the maintenaince part.