The most important factor in comfort is the fit. You can usually raise or lower the handlebars on a bike to get them where you want them. You can even change the stem to bring them closer or push them out further. A bike will have a certain range of adjustment. So it is not going to be the bike itself in the end, but how you have it adjusted.
If you are riding with your torso approximately horizontal, then a big bump that jolts your pelvis up, that will put a shear stress on your vertebrae. Sitting up vertically, that bump will become compressive stress. Which do your vertebrae prefer?
In the limit, the fastest bike is not the most comfortable bike. There is always a bit of compromise.
There is also budget. The bikes I dream about are up in the $5000 range. To spend money like that, even if you can afford it, you really should know quite exactly what you want. If you are just trying to sort things out a bit, spend more like $800 which can buy a very nice bike, good enough so you can really see if the general set-up is what you want. That $800 could work for decades. Or maybe after 3 years you have figured out what you really want and then it could make sense to spend the big bucks.
Handlebar styles are a real blast! These days people are experimenting with all sorts of shapes! Straight flat bars don't make much sense. But look e.g. at something like this:
http://www.rivbike.com/product-p/hb3a.htm
Have fun exploring and learning!