There are mainly two editions : STANDARD and DATACENTER.
Apart from feature sets, the main difference lies in virtualization rights. So, choosing between the two will be based on your virtualization strategy. If your strategy depends on a highly virtualized environment, Datacenter Edition would be an appropriate choice, as it provides you with optimum flexibility with its unlimited capacity for virtualization. If your strategy depends primarily on a physical environment or a lightly virtualized environment, Standard Edition may prove to be a cost-effective option.