For the virtual machine, there is no practical difference between assigning multiple processors or multiple cores. This option was added several versions back, to make it easier for people licensing certain OS's or applications - some are licensed per socket, others per core. Performance wise, there should be no difference - the virtual machine will still use one CPU resource per processor/core you specify.
Note that you should never assign more than (n-1) virtual CPUs to any VM, otherwise you may starve your host. In this environment, typically less is more.