Getting started... which version of desktop to match production target?

Hello!

I just started my developer subscription trial, and I'm slightly confused as to which install I should download, mainly because I'm unsure of compatability levels. 

My goal is to setup a local dev/test environment that I can use, which matches a production target that is currently running RHEL 5.6. I plan to install an RHEL 5.6 server on my vsphere host for deployment testing, but I'm unclear if I should also run RHEL 5.6 on my developer workstation, or if I should install 5.9, or 6.4? (workstation, or client? what's the difference?)

Any pointers in the right direction would be much appreciated!