They are identical in swift. If you look at the definition underneath, you will see this:
public typealias CVPixelBuffer = CVImageBuffer
which means that you can use you can use the methods here if you want to find the image planes(I don't know what that means exactly). But CVPixelBuffer is a concrete implementation of the abstract class CVImageBuffer. There are two types of CVImageBuffers in use at the moment-CVPixelBuffers and OpenGLBuffers. You can choose to break the abstraction if you know for certain that your CVImageBuffer is a CVPixelBuffer.