Simple answers really....
Megabar is a product designed more for uplighting and wall washing due to the strength of the 10mm LEDs and the narrow beam projection angle.
The Megapixel uses 5mm LEDS which creates a more "visual" effect, so are ideal for facing and pointing towards your audiences on stage, or truss etc. The effect from the Megapixel is ideal to use for running chases, blinder effects, funky effects and general "disco" colour projection. The effect tends to have a wider beam angle too, as the 5mm LED isnt as direct.
All depends on the useage you want the product to do. The Megabar LED is also split into 3 LED RGB Segmants per bar, so you can make the effect slightly different and more exciting....not as good for visual applications though as the Megapixel.
If you want an LED bar effect to do both visual and wash effects, then thats where the Megabar Pro comes in.