I take cuttings for two reasons, increase the number of plants from a special variety and to get a good root system started.
As for the difference in timing, if you plant a cutting, with say four sets of leaves, and stop the main growth tip, it will bloom earlier then a tuber. Why? The tuber has to emerge out of the ground, get four sets of leaves and then be stopped.
Now to really throw you a curve!!! I also grow for show. If I have a show on Sept 14, 2008, and I want A size (8"-10") dahlias in full bloom for that date, I'll count back 60 days and stop the main growth tip on the plant (tuber or cutting). It dosen't matter if there's eight sets of leaves (that is possible if using a cutting) or four sets of leaves from a tuber. The number of days to bloom will still be 60. I'll only allow four laterals to develop. So if that cutting is 3' tall, it's almost stripped right down to ground level (I only want four laterals and lower to the ground they are, the easier it is for the plant to supply nutrients). If I don't do this, they get too tall.

If you are growing for cut flowers, I'd suggest that you use only small dahlias (BB - 4"-6"), waterlily's, ball and miniature dahlias. If using cuttings and you take out the growth tip say by May 14, and then plant if conditions are okay, you can have blooms by the first or second week of july.