heavier means slower? Not really, not in the price range bike you are talking about, and not at your size....
you will not really see much speed difference in the lightest and heaviest bikes in that range....
what you need is something reliable and comfortable...what ever you like and will ride will make you faster because you will ride it more... and further develop the motor. at your size the material the bike is made of will not be a factor worth thinking about when it comes to speed.