The 24 Nosler is a shorter cased version of their 22 Nosler. Nosler went with the shorter case so you could seat the heavier longer 6mm bullets out and still feed them through an AR magazine.
The 6mm DTI has a little more case capacity than does the 24 Nosler but the case is longer, so in an AR build where you are limited to OAL by the magazine, once you seat the bullet to mag length you'll be filling up the extra case capacity of the DTI with bullet, so I'd imagine the velocity between the two is going to be pretty close.
