A lot of people ask about replament window; here are my thoughts: I've installed over a thousand in my career, mostly made by @harveybuildingproducts. If you're concerned about cost and don't mind loosing visible glass they are not bad.
First, you need a quality window, not an American Craftsman off the shelf at Home Depot. I wouldn't install anything under Harvey's classic series, their "better" window. If you can afford it, look into a @integritywindow. It's a bit more, but you have a lot stronger of a frame and a lot more options.
Second, please hire a professional. You can have the best window made, but installed poorly it will be worse than what you're replacing. A poor install not only reduces the energy efficiency, but it may also cause water penetration issues.
Third, think about the exterior of your home. If you're planning on siding in the future, do the projects together. If you can't do the windows first, then do siding. Siding first will never look as good in the end. If you decide to do windows and siding together I usually recommend new construction windows though.
So basically, preformance wise new construction and replments installed correctly are very similar. New construction windows require more labor, so that adds more cost. With new constructions you may end up with more glass than current. Replacements you are going to lose at least 5" in the width and height, maybe more depending on the manufacturer.