The Facts:
The weight difference between 303 and 404 is about 50g - that should not be your decision maker.
The 303 FC rear wheel is 8% stiffer than the 404 FC rear wheel. I'd say that is a huge difference. If you're a heavier rider, push taller gears, are very powerful, or spend a lot of time climbing, the stiffer wheel is just going to feel better.
The 303 has a more consistent drag profile in relation to crosswind angle than the 404. While the 404 has a lower average than the 303, it is also much more variable, with significant dip at 15 degrees. The 303 actually has significantly less drag in crosswinds greater than 15 degrees from front.
303:
404:
This is an important distinction because the center of pressure on these wheels (or most other wheels) is not centered on the steering axis. With the FC wheels, that center is closer but still not perfect. In constant crosswinds, you very quickly adjust, but in gusty winds or extremely high speeds (like a long steep descent where you might be closer to 45+mph), you would experience more rapid fluctuations in that steering torque and thusly require more attention to handling. Since the 303's have a more consistent drag graph, they also have a smaller fluctuation in steering torque as crosswinds vary in direction.
My Experience:
Personally, when I switch from my low profile 7850-C24-CL wheels to my 404 FC-Clincher wheels, the handling is very different in crosswinds. I wouldn't say it's unmanageable, but it definitely gives me pause when starting down a long downhill or when riding in gusty conditions that cause that torque to fluctuate a lot. I am far more willing to attack downhills with the low profile rims primarily because they are slightly more predictable.
I work in a shop that sells a lot of Zipp wheels and we've done demos. Having ridden the 303 FC Clinchers back to back, the 303s definitely have a more predictable feel to them when descending in crosswinds.
So, here it is:
lots of climbing/descents, heavy/powerful rider, or lots of crosswinds, go with the 303.