How much more do you weigh than your wife? From my Pinewood Derby experience I can tell you heavier objects roll downhill faster than lighter objects.

The 700 vs 26" debate gets tossed about quite a bit. In the real world,there are many factors that determine how fast a bike is beyond the wheel diameter. The rider is the biggest factor. There's also the weight of the bike,the weight of the wheels,quality of the hubs/bottom bracket,gearing,condition of the drivetrain/hubs/BB,tire width/pressure/tread/weight,riding position...lots and lots of things. The main reason I have 26" wheels on my commuter is because it came with them. The second is the availability of tires. Every shop in the area carries 1.5" slicks for spring/summer riding on bad roads,and 2+" treaded/knobbie tires for fall/winter snow and goo. Wide,38+ 700 tires are available,but you have to special order them or go on-line.