I agree with Rowan, smaller riders need smaller frames which work best with smaller wheels. Ther difference in efficiency between large 700c and small 26" (MTB) is pretty small if using similar style tyres. Many professional cycle couriers use MTB size wheels and they need to be quick.
The ideal style would be a lightweight non-suspension frame using a high pressure road tyre (1.5" wide slick).
Light bikes are easier to handle and ride.

There is one "hybrid" bike for smaller people which uses the MTB wheel: the Terry Susan B. This is just about perfect for everyday use but slightly higher than your budget. It comes with smaller components as well, a factor often overlooked by major manufacturers. I'm not aware of any other reasonably lightweight hybrid taking MTB wheels.