I think in some stance they are similar. A couple months ago, AppSync supported only AWS Dynamo DB. Do not if it is the case for the moment. I do not think also that AppSync will give you that much GraphQL experience as Prisma can. I do not think it is possible to run nested mutation with AppSync. Be aware also that with AppSync you need to learn VTL (velocity templating language)
http://velocity.apache.org/engine/1.7/user-guide.html#what-can-velocity-do-for-me