vMA is basically its the missing service console for ESXi. When you have multiple Hosts to manage and you don't want to ssh/ login to the machine each time, you can make use of vMA. Admins who are more familiar with linux environments uses vMA.
PowerCLI is the PowerShell extention for VMware products. Admins those who are more microsoft oriented mostly uses of this FREE tool. PowerCLI can also be used to develop tools with a GUI and there are a lot of script editors that supports PowerCLI, like PowerGUI, PowerShell Plus etc.
~dGeorgey
