Apart from personal preference, and desire. . . . .I don't really see the advantage of a lot of whippet x lurchers that are 1/2 or 3/4 whippet, working wise. . . . in comparison to a straight whippet.
If getting a lurcher, i'd always be looking for some grey blood, to bring the size and the top end speed up.
Having said that, if it's what you want, get one!
And for the record i've seen a wicked little bull/whip, one of powells pocket rockets, and she was a 21/22 " powerhouse, and nippy as feck! And good at her job.