Well, we know neutron star binaries exist, so we can make estimates of the rate at which they merge. We suspect black hole binaries exist, but we don't have good statistics on how many of them they are, so these estimates are more educated guesses. If we knew all of the answers, there would be no need to build the gravitational wave detectors! But, having said that, my guess is the first detection will be a neutron star binary merging to form a black hole. But we'll have to wait and see. We should have unambiguous GW detections once Advanced LIGO comes on line, which is expected to be ~2015.