Right now, I've got Marathons in the 38mm size instead of Gatorskins in the 32mm size on my Sojourn. There is a noticeable difference, and I feel like I'm riding tractor tires through peanut butter all the time. This makes me want to try some tires with less rolling resistance than Gatorskins, an idea I didn't think much of before trying this.
I can't tell if it's the difference in brand/type or difference in tire size or both, though.