Being everything equal, the 700c is in theory and in practice faster. To which extent? It depends on the cyclist. Again, everything the same, 700c has lower rolling resistance and higher rotating momentum (both of them good for keeping high speeds, but not so much for accelerating); 26" has lower wind resistance (only for the front wheel; the rear one is behind wind drag of seat tube for all bikes) and weight, and lower center of gravity (better for quick and nimble swerves and wriggles). The outcome being that speed-wise the advantages of the 700c slightly outrun the 26"'s ones. I've ridden both, and the 700c is usually 1 to 2 mph faster with the same effort. Anyway, if you have to keep up with a group of fitness-matched people, wheel diameter won't matter much.