Advantage of using Ubuntu Enterprise Cloud (UEC) compared to VMWare?
UEC does employ KVM virtualization however! I would think VMware's best competitive advantage is the availability of mature GUI management tools, and UEC's best competitive advantage is being open-source, and providing a compatible EC2 API interface
UEC is not a visualization suite. VMWare is.
http://www.ubuntu.com/cloud
UEC allows you to creates a cluster of machines and distribute load across them. VMWare allows you to create virtual machines to take advantage of powerful hardware.