MySQL 5.1.x: Performance improvement in Enterprise edition Vs community edition
I don't think you can get a MySQL Enterprise edition for v5.1.X version.

You can see the diffrence in performance between Community and Enterprise version here:

http://www.mysql.com/products/enterprise/scalability.html

You can also download Enterprise product guide whitepaper to know more details.

In summary additional features in MySQL Enterprise version are MySQL Thread Pool, MySQL Enterprise audit and a Technical support for your server related issues.