Why not have two bikes for commuting?

Besides, always good to have a back up in case one is down for repairs or an early morning flat.

To answer your question, I actually prefer drops bars and skinnier tires over a heavy steel MTB with 26x1.5 slicks. I like the varied hand positions that drop bars provide vs a flat bar, too.

Really though, your choice may have more to do with where you ride. And with an 8 mile commute, time difference will be minimal.