Very seldom will anyone use the 11, so given the choice, I'd go for the 12-26, since you're more likely to need the help going up a hill - (not that there's much difference between a 25 & 26). If you already have a bike, then you'll soon find out for yourself if you have a low enough gear for the hills near you. If you're shopping at a Local Bike Shop, then perhaps they can make a recommendation based on your fitness, size, terrain etc .... Cassettes are easy enough to change if you have the wrong one.