the skinnier tire will no doubt be the faster one (with tire pressure being equal).

but i think the true benefit of the skinnier tire is the ability to pump it up to a higher PSI which will really make you faster.

especially if your just riding pavement i say go for the skinniest tires you can find with some decently thick sidewalls and puncture protection.

i ride a commute mixing road with cinder path trails on WAY skinnier tires pumped up to 100 psi on my crossbike.

higher PSI makes you FASTER!!!

as for front and back on my mtb's i always run a bigger tire in front with lower PSI than my back tire because i need more traction for cornering. i'd say for commuting it shouldn't really matter to much one way or the other. but i like the look and feel more secure with my bigger tread in front.