How to perform Windows VCenter Upgrade with less or no issue at all ?

Hi,

I have 24x VMware ESXi servers 5.1 Update 1 on HP Blade Servers managed by my Windows VMware VCenter VM that is currently on version vCenter Server 5.1 Update 1 (build 1064983 2013-04-25) as follows:

Windows Server 2008 R2 SP1 Standard - 1x vCPU & 10 GB vRAM

Microsoft SQL Server 2008 Standard Edition (64-bit) (SP2) - 10.0.4000.0

My question is that which of the below VCenter version upgrade is less stressful or with no issue at all:

VMware vCenter Server 5.1 Update 3a and modules

VMware vCenter Server 5.5 Update 2e and modules

Note: Since the VCenter and SQL DB are in one single place, I have clonedto template the exsiting running VCenter server for easy roll back by deploying from template when connecting directly to ESXi where the template is running.

Thanks in advance.