As someone who has done this path many times let me provide two different theories:
1. Upgrading to 5.1 is a pain due to all the SSO issues you will run into
2. Upgrading to 5.5 during the first few weeks of release could be more of a pain.
At this point if you need server 2012 then I would go the 5.1 path because at least all of it's issues are known and well documented / supported by VMware. I never want to take my production environments into a new software release day one. I will be looking at the 5.5 release around Feb. 2014 by then all the issues will have been worked out. That being said I am running 5.5 beta right now on a test cluster but not in production.
So simple answer if you have not yet updated to 5.1 (because you don't like the bleeding edge) why go bleeding edge on the next update.
Thanks