As mentioned above whatever bike configuration you will get on is what will work. However I transitioned from one to the other so I'll tell you why, etc.
I rode more of a flatbar at first because....
- The bike I bought had the flatbar. I bought the bike because it was beefier
- I HAD to ride more upright because getting into the drops was very difficult because my belly was in the way. It would hinder my breathing.
- Off the shelf it had beefier rims
- I didn't know enough to know that I could have modified (handlebars, rims, etc) a road bike to do all that
I lost some weight and showed myself I was going to stick with it so I got a road style bike. Why?
- I liked doing between 30-40 miles and wanted to go faster. Sure it's "the engine" but when "the engine" is running a more efficient machine you go faster/further
- When I got the bike I still couldn't get in the drops comfortably though it was WAY better than when I started. I kept working and now I can ride in the drops
- My road style bike is lighter and I tended to not "mash" as much. I could work the entire spin and stop furthering an imbalance I was developing. I could have worked on that with my heavier bike but I suck so I didn't
- With my drops I have a few more hand positions to get into to keep the hands comfy for longer
- Being in the drops and generally bent over more takes a bit of pressure off my rump