bull is much stronger and more determind even for a rabbit thats close to the hedge,were as colly x might stop and think ,should i or shouldnt i?but my personal choice and also my way to split the two breeds and turn it into one, would have to be the irish wheaton terrier x.i have an irish wheaton x grey hound xsaluki x grey hound and to sum him up would be.quick,excellent feet, intelligent,hard working, honest,loyal and very game,and a joy to go out with.