The heavier bike will make you slower, since rarely any ride is perfectly flat nor uninterrupted.

The fatter tires drags you a little bit more, and it becomes more noticeable as the ride gets longer. The bigger culprit is on wind resistance, pushing air is a harder job than it seems at first when your top speed is 10 miles an hour.

I have a comfort bike too, and I only use it in certain trails that call for the 38 mm tires.