Note: Discussion successfully moved from VMware vCenter™ to vSphere Upgrade & Install
With a fresh install, you will lose the performance data, permissions, roles, customization specifications and other vCenter specific settings.
(1) This is a good point - you can install vCenter Server 5.1 and all of its components and verify that everything is working, before adding any hosts to it.
(2) The db move is also simplified by going with a fresh install.
If you think you can live with the loss of the above-mentioned items, then the fresh install is really best with the new database you want to use.