vCLI mainly provides remote versions of the esxcfg-* commands, plus some other utilities. If you were familiar with that environment, you may find that vCLI is easier for doing ESX host configuration. However, vCLI does not offer much outside of host configuration.
PowerCLI takes a very different approach and provides a PowerShell interface to vSphere. It also supports managing the entire vSphere stack: storage, network, ESX, VM and guest OS. PowerCLI gives complete access to the API so within PowerCLI you can do anything that vCLI does. The reverse cannot be said, many things that PowerCLI can do cannot be done with vCLI.
vCLI runs on both Windows and Linux. PowerCLI is Windows-only.
They are both tools aimed at admins.
Hope this helps answer some of your questions.
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Carter Shanklin
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