Ditra is made by
www.schluter.com and is a great product. It is designed for combating deflection issues, not potentially spreading that can come from a crack. Typically, crack isolation membrane is used over and to the sides of the crack by maybe a foot or so. Ditra is designed to be used in place of cbu on a wood floor, or over a slab in its entirety. On a slab, it has an additional advantage in that the channels allow the slab to breath a little, and is a great choice over a green slab (which shouldn't be tiled otherwise for at least a month). An isolation membrane is typically quite thin but elastic. Ditra is fairly rigid and much thicker than an crack isolation membrane, making its use over only a crack problematic. It is conceivable that you may want both. Ditra does have some abilty over a crack, but that is not its primary function.