Hmmm, mountain bike or fixie. . . two popular commuting options that I think make mediocre transportation bikes.

I would chime in for a cyclocross or touring bike instead, but if I had to choose, I'd go with a mountain bike with no suspension anything and road slicks installed. I think that's way more versatile than the fixie and at least then you have a decent mountain bike for trails.

(I'll admit I'm a bit of a curmudgeon about fixed gear outside of velodromes!)