API
Nova features a pluggable architecture for API support. The current Nova incarnation supports:
OpenStack API 1.0 (with preliminary OpenStack 1.1 API support)
Amazon EC2 API
Many customers will probably request the mostly compatible EC2 API interface, which supports about 90% of Amazon’s current implementation. However, the OpenStack API (especially the 1.1 version) will probably be the more widely implemented version in the long run, as it is an open API not controlled by a single company.
While there is no technical requirement to pick one API over the other, it will be confusing to your users if they need to use both. Very few users tools or libraries will support both and allow them to switch on an API by API call basis.