The difference between any two bikes, performance wise, is small.
In my tri club we have people riding along with mountain bikes now since it's winter.
these guys have no problem doing 70 miles at an average of 17 mph keeping up with the rest of the pack on racy roadbikes.
They sometimes even take the lead and I quote: "to get a better workout".
More than 90% of the difference is in the rider, not the bike ... so unless you're racing and aiming for those few percents extra ... ride whatever you feel like.