Fit is the most important thing - make sure you know what size you need before you buy. you can tweak it a little with different length stem, but it needs to be the right basic size. You can get a professional bike fit at a store and they can tell you what size you should be looking for. I would start shopping with that piece of information. worth paying for if you need to.
Also, the handlebar style can be a very strong personal preference - for me i really need drop bars, other people like flat bars better. it would be useful to think about those things before you buy.
good luck with you shopping!