The differance in real world commuting speed between a 700 and 26 inch wheel is nothing. Even gearing really doesn't make a differance unless you're a really strong rider and you're not in too much traffic. I avoid hammering on the commute because I have traffic, weather, darkness and lots of work related stuff on my mind. But here are a couple of real world things to think about.

I'd rather flat with a 26 in wheel with a 1.5 inch slick than a road bike wheel with a super skinny, high pressure tire any day.

26 inch wheels with 135 mm rear spacing are way cheaper than road bike wheels (700 with 130 mm spacing) As a general rule, MTB bike parts are cheaper and stronger than road bike parts (and heavier, I know)

For the money, 26 inch is better deal if your commute isn't too long. Think about a wide range MTB style casette in back and a double chainring up frount without index shifting. You would win any races, but it's a great commuting/touring set up.

But nothing's like hammering on a road bike