hobie 16's are and always will be faster downwind, they just have more sail area, pretty simple, i disagree with matt. its more a matter of sailing/boat handling skills, tuning and weight, if the 16 is not winning downwind they probably aren't all that good. With that being said, having sailed both classes i think its easier to be more competitive sailing a mosquito than it is in a 16.
My results at nationals show this, in 16's i was mid/back of the fleet, and in my first mossie nats i came 3rd, so if u are looking to get into either class and want to be competitive u will have more chance of doing that in a mosquito