The 52cm flat bar may feel best to you because you are probably sitting more upright due to a longer head tube & bar position. The correct fit will depend on several factors--not just your physical dimensions, fitness level and flexibility, but intended use of the bicycle. Are you interested in long rides and centuries, touring, club rides, racing, or just leisure rides around the neighborhood? Sitting upright may be great for cruising, but not for fast club riding or racing. Riding a bike that does not fit can lead to pain and overuse injuries.

Indysteel said it best when she recommended seeking out a professional fitter. The best ones aren't cheap, but you will save yourself money and frustration due to ill-fitting bike(s) in the end. And not all stock bike's geometry match well with the intended rider, especially women with our "non-standard" proportions. Getting the correct saddle position over the bottom bracket is paramount in determining the right frame size; reach and stand-over are the next things to consider. When all is dialed in and adjusted to the type of riding you want to do, you will have a properly fitted bike. As you gain fitness and flexibility, you may wish to stretch out more and/or go lower with a simple change of the stem.

It seems like a lot to think about, but it will make your riding experiences so much more enjoyable. You can also check out www.competitivecyclist.com for a free on-line fit. Not quite the same as an actual fit session, but good to see if you are in the ballpark.

Good luck!