Vmotion moves a running VM from one host to another. (VC Client GUI, means by "Migrate...")

SVmotion moves a running VM (files belong to that VM) from one vmfs storage to another. (have to be done through CLI. No VMware supported GUI so far.)

Both should not inturrupt VM. VMs can be up and running during the process.
Note: If you can power off the VM, you can use VC Client GUI ("Migrate") to move both!

The most famous Third-Party GUI tool (a VI Client plug-in) for S-Vmotion recognized by VMWare is developled by Andrew Kutz: http://communities.vmware.com/thread/126141