When I bought my Specialized hybrid...it came down to chosing between a W's 53 and a M's 51. I wanted a step through. The clerk, at a very reputable LBS, told me I was fine on either.

HOnestly, I felt a little better on the smaller bike but was worried about the top tube so I got the W's....and I think it's just a little bigger than ideal...

I discovered this when I bought my Jamis...a 47...and I am just SO much more comfortable on it.

a couple of centimeters in the wrong place can make a BIG difference.