Not an expert, but in my dabbling I've found that tokens work great for api and from javascript to api, and traditional cookies lean mostly for a ui. Either or both will work depending on what your trying to do.
You can follow something like this link that does cookie for the ui and token for the api http://blog.iteedee.com/2014/03/asp-net-identity-2-0-cookie-token-authentication/
app.CreatePerOwinContext(ApplicationSession.Create);
app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
// Token Authentication
app.UseOAuthBearerAuthentication(new OAuthBearerOptions());
I think you can set the cookie authentication options authentication type to bearer if you want bearer for both, but you would have to play with it. The token would be in the owincontext under ".AspNet.ExternalBearer".
I also think if you register the Identity 2.0 middleware i think it also registers the oauth middleware stuff so you don't need to register the oauthserver middleware yourself. Thats the OAuthAuthorizationServerOptions code you posted. You dont need it.
if the ui and api are in separate then its a bit harder if you want to do some sort of single sign on from the ui pass to the api. I would recommend looking at opensource identity server or authorization server from thinktecture.
If your set on owin middleware and Identity 2.0 you would need to make sure the token can be read by both application and api and you probably would need to implement ISecureDataFormat. But remember, decryption doesn't mean you can 100% trust a token, it should be signed and verified. Depends on your needs.
Sorry, I guess thats a long ramble... Good luck.