I'm speaking very generally here, but sitting straight up can be comfortable for quite short periods. Probably the most upright are sit-up-and-beg bikes, and if you go over to Holland you'll see 70- and 80-year-olds riding them. The downside is that they're not the fastest bikes around.
At the other end of the scale, take a look at professional road racers. These guys are as close as possible flat down to the bike. Efficient, but mightily uncomfortable.
The reality for the rest of us is that something in between is the most comfortable, but our geometries all tend to be different so different styles suit different people.
I think the good thing is that, based on what you say about distance, speed, terrain, climbing, load etc., your requirements aren't very demanding, so you'll have plenty of choice of styles so you should really be able to find a bike that is comfortable for you. (This also opens up the second-hand market, if you wish.)
I've ridden in London and I assume NYC is similar - be prepared to get a couple of decent locks, and make sure the bike you get will be able to carry them around somehow.