Are you comparing apples to oranges? My mountain bike has a 19" frame size (perfect for me, I'm 5' 6") and 26" wheels (which are standard on mountain bikes and some hybrids). Kids bikes are sized by wheel sizes. My road bike has a larger frame (mens M - can't remember what that translates to in inches) and bigger wheels, because it's for road riding.
The only way to really know what fits you is to go to a bike store and try the bikes they have in the type you want. You don't have to buy from them, although I know the bike stores here have used bikes also, once you know what you want then you can look online. I'd go for more gears if you plan to use it in hilly areas - both mine are 21-speed and although it sounds a lot I tend to use all the ranges.
I agree with the saddle issue witchyonadiet mentioned - especially if you buy a men's bike. I have women's saddles on both my bikes and they are wider and waaaaaay better for me!