So the 12-27 will give you some really low gears. Which you should go with depends on the type of terrain you are on and what your cadence is while climbing. I think Carmichael recommends something like ~80rpm and to pick your gearing based on what will enable you to ride like that in the hills. I know people who are plenty strong enough to ride in a standard double, but either have a triple or compact so they can have really low gears for when they do short steep stuff of a grade similar to what you find in S.F.