For an environment that size and if SQLEXPRESS is the DB I would choose an in place upgrade, however if you are on 4.1 make sure the database is on a supported version such as 2008 R2 for 5.5.
Additionally you could always deploy the Linux Appliance which runs an embedded database and does not require any microsoft licensing, the draw back is that vum would need to be installed on a windows server.
As for performance metrics you can export those out of vCenter to keep as a record should you choose a new stand alone vcenter, or vcenter appliance