Prime lens.

Youve probably been given a hard time by some of the M43 crowd here I apologize!

To answer your question - its no question that Olympus can make great zooms - Look at the Zuiko 4/3 SHG range rated as some of the best in the world, and the sharpest. Look at the 12-60. Some would be hard pressed to tell the difference quality wise compared so similar HQ primes. But I would think the primes would still have a slight advantage, due to less complexity and exotic components required in the zoom. Also they can be made smaller with faster aperatures, Look at m/43 75mm and I dare you to find a zoom in any format sharper at 75mm ( 150mm).

That being said, however - I think even olympus has problems making HQ zooms Small. Panasonic have done a decent job with their recent zooms, and I am sure more are to follow.

But i think the main reason is SIZE, followed by the massive cost to miniturise these lenses....