Installation via Appliance vs. ISO image?

vCenter Server 5.5 Update 2b
  and Modules

VMware-VIMSetup-all-5.5.0-2183112-20140901-update02.iso

Installer for vSphere Single Sign On, VMware vCenter Server,
  VMware vSphere Client and Web Client, VMware vSphere Update Manager, VMware
  vSphere Update Manager Download Service, VMware vCenter Orchestrator, vSphere
  ESXi Dump Collector, vSphere Syslog Collector, vSphere Auto Deploy, VMWare
  Log Browser, and VMware vSphere Authentication Proxy. NOTE: vSphere Single
  Sign On, VMware vSphere Update Manager, VMware vCenter and Web Client
  requires 64-bit capable servers for installation and execution.

vCenter Server 5.5 Update 2b
  Appliance

VMware-vCenter-Server-Appliance-5.5.0.20200-2183109_OVF10.ova

OVA file for the VMware vCenter Server Appliance. Use the
  VMware vSphere Client to import this .ova file to your setup. NOTE: Please
  make sure to save this file locally with the .ova extension

vCenter Server 5.5 Update 2b
  Appliance – OVF File

VMware-vCenter-Server-Appliance-5.5.0.20200-2183109_OVF10.ovf

OVF file for the VMware vCenter Server Appliance. Use the
  VMware vSphere Client to import this .ovf file and the related .vmdk files to
  your setup. NOTE: Please make sure to save this file locally with the .ovf
  extension

vCenter Server 5.5 Update 2b
  Appliance – System Disk

VMware-vCenter-Server-Appliance-5.5.0.20200-2183109-system.vmdk

System disk file for the VMware vCenter Server Appliance.
  Download this .vmdk file in the same directory as the .ovf file above.

vCenter Server 5.5 Update 2b
  Appliance – Data Disk

VMware-vCenter-Server-Appliance-5.5.0.20200-2183109-data.vmdk

Data disk file for the VMware vCenter Server Appliance. Download
  this .vmdk file in the same directory as the .ovf file above.