Well, the 26" wheels are stronger because the spokes length from rim to hub is shorter. Stronger does not necessarily mean 'better' unless you have very rough roads in combination with a heavily loaded bike which can cause spokes to come under greater tension than 'normal'.

A 29'er is not necessarily 'faster' than a 26incher, or even a 700c wheel. Speed is determined by gearing, tire choice, aerodynamics, rider, and things of that nature.

The advantage of 29" wheels over 26" wheels is that the larger wheel can more smoothly roll over things like potholes or rough terrain. To illustrate, imagine rolling over a pothole with a bike that has 20" wheels. It would be jarring because the wheel is 'sucked' into the pothole and has to sort of 'climb out' again. With a larger wheel, less of the rim/tire is 'sucked in' so the effect is a smoother ride. Of course, over good terrain everything is smooth.