The big benifit of ESX over ESXi is with ESX you can use Virtual Center to manage everything form a single location (Virtual Center is an additional one time cost).
If you are using SAN storage, you can use Virtual Center to create an ESX cluster which will dynamically move guests from one node to another as needed for performance without any down time (you can control how aggressively this is done).
With ESXi you can't use Virtual Center to manage the host so you have to manage each host independently. With 1 or 2 hosts this isn't that bad, but as you get more hosts it becomes a pain having to remember which host a VM is one. With Virtual Center you group everything into a datacenter and everything is presented as a single resource pool.
In addition with Virtual Center comes a license for the VMware converter, and the ability to create templates which make creating new VMs a 5-10 minute process to spin up a new Windows VM with the OS already installed.