My experience, worth what you are paying for it...

My rain beater is an aging hybrid with straight bars, while the
bike I would rather be riding is a conventional drop bar road bike
affair. Since the weather has been wretched in Columbus, Ohio for
3-4 weeks now, I've been riding the hybrid almost exclusively for
my daily commute. I got the road bike out twice this week, and
noticed after 20 or so miles that I was feeling some stress in the
lower back muscles. They weren't accustomed to participating in
the pedal strokes so much after weeks of a more upright posture.
The feeling went away for the 2nd ride on it, leading me to believe
that the road bike posture is ultimately better for me, as it invites
more of my musculature to the pedaling party.