Mountain Bikes: they are sturdy and designed to go everywhere including off-road. Good for rough riding. In general heavy and with deep thread tires slow on paved roads. Not good city bikes unless narrower and slick tires are installed.
6 speed: easy to use and perfect for the city and bike paths. There are enough ratios to gradually build speed without forcing too much. Not great for steep hills, ok for medium grade climbs. - Derailleur: Shimano SIS
Tires 26 x 1.90: wide tires, deep thread, all terrain.