I have used both and Mothers is better by far.
Meguairs will streak like it's going out of style, I have a puddle of QD on my car and the clay bar made long thick hard to remove streaks, I did half of the rear spoiler of my car and just tossed the stuff in the trash. Until they have some sort of "new improved" clay bar I'll never buy it again. I had to use polish to remove the streaks. Every other stroke with the bar was a streak, at first I thought it was normal and it would come right out ....
Mothers is awesome, it pulls everything from old bugs to spray paint from a car and it does all of this with very little rubbing action. If your car is new I would think you could get away with 1/4th of a bar for the entire car. My car was never clayed and I had to use half of a bar (as an example).
Mothers all the way. Too bad you can only find it in an expensive kit (18.00).