From an ESX perspective, 1 vCPU with 4 virtual cores is exactly the same is 4 vCPU with 1 virtual core. The ability to provision cores to a VM is more about what is presented to an operating system. If you want to run Windows 2008 standard, you are limited to 4 vCPUs. With the availability of cores, you can configure 4 vCPUs with 4 virtual cores, thus giving the VM 16 cores worth of processing power. The KB below provides more complete information:
VMware KB: Setting the number of cores per CPU in a virtual machine
