With my first bike, an ATB/"mountain" style bike felt better...I think that's common for the inexperienced cyclist. It's more familiar feeling, being somewhat similar to your childhood bike. The upright position is quite reassuring to the "new" cylist.
However, time on the bike eventually brought greater fitness, skill, and distance. And with all of those, the upright "comfort" of the ATB was no longer comfortable, and the tires and gearing had reached their performance limits. I still ride this bike from time to time for short casual rides, but not nearly so often.
So I had to get a touring style bike for my rides and commutes. Sometimes I think I should have started with that bike to begin with, but other times I wonder if it would have seemed...daunting. I just don't know. I do know that 90% + of my riding is on my touring style commuter. With longer rides, a less upright position is actually MORE comfortable, not less.