definatly a quad bike!! :P but afterall it depends, speed or comfort.... there are lots of factors that can change your mind, eg: safety, enjoyment, use of vehicle, price range and which you prefer...
i really does not matter your age the 49cc bike has to be 21 to 24 inches in height they are not to be road on main road or streets i have a 49cc of my own i ride were ever because they are legal in...