PTP and MTP are both protocols for transferring files from a digital still camera (DSC). The protocols provide no functionality for triggering new images, or setting exposure control. You will be stuck using the camera specific SDK. If you want multiple vendor support Nikon has an SDK that provides similar support as the Canon SDK for their cameras.
More info on PTP and MTP can be found here: http://en.wikipedia.org/wiki/Picture_Transfer_Protocoland http://en.wikipedia.org/wiki/Media_Transfer_Protocol
I forgot to mention that WIA may be interesting to you, assuming your camera’s driver provides a WIA interface.