May I knwo the difference between them? Especially from the design purpose perspective. When would we choose OTV and vice versa.
There have been many, many posts on the Internet on this topic. For starters, Scott Lowe has written a good "vs" article here -
http://blog.scottlowe.org/2011/12/22/otv-and-vxlan-layer-3-connectivity-compared/
As OTV is limited to Cisco technology, your design is limited to a Nexus 7000 if you go that path. A good whitepaper on OTV can be found here -
Where VXLAN is a standard (that Cisco is also involved in). Some details on VXLAN here -