Hi I would go for ESXi 5 if you can, as I have found the there is an increased stability in the virutal machines and a lift in performance since the footprint of the hypervisor has been dropped a little.
The only drawback I would caution with regarding the free ESXi 5, is the patching mechanism, which is clumsy without having a vCenter 5 license which allows you to run 'Update Manager'.
Mark
